docs: move CONTRIBUTING (#2402)

* docs: move CONTRIBUTING

* docs: clarify PyPy promise

MANIFEST.in

 recursive-include include/pybind11 *.h
-include LICENSE README.md CONTRIBUTING.md
+include LICENSE README.md .github/CONTRIBUTING.md

README.md

@@ -8,12 +8,11 @@
 [![CI](https://github.com/pybind/pybind11/workflows/CI/badge.svg)](https://github.com/pybind/pybind11/actions)
 [![Build status](https://ci.appveyor.com/api/projects/status/riaj54pn4h08xy40?svg=true)](https://ci.appveyor.com/project/wjakob/pybind11)
 
-**pybind11** is a lightweight header-only library that exposes C++ types in Python
-and vice versa, mainly to create Python bindings of existing C++ code. Its
-goals and syntax are similar to the excellent
-[Boost.Python](http://www.boost.org/doc/libs/1_58_0/libs/python/doc/) library
-by David Abrahams: to minimize boilerplate code in traditional extension
-modules by inferring type information using compile-time introspection.
+**pybind11** is a lightweight header-only library that exposes C++ types in
+Python and vice versa, mainly to create Python bindings of existing C++ code.
+Its goals and syntax are similar to the excellent [Boost.Python][] library by
+David Abrahams: to minimize boilerplate code in traditional extension modules
+by inferring type information using compile-time introspection.
 
 The main issue with Boost.Python—and the reason for creating such a similar
 project—is Boost. Boost is an enormously large and complex suite of utility
@@ -26,19 +25,18 @@ become an excessively large and unnecessary dependency.
 Think of this library as a tiny self-contained version of Boost.Python with
 everything stripped away that isn't relevant for binding generation. Without
 comments, the core header files only require ~4K lines of code and depend on
-Python (2.7 or 3.x, or PyPy2.7 >= 5.7) and the C++ standard library. This
-compact implementation was possible thanks to some of the new C++11 language
-features (specifically: tuples, lambda functions and variadic templates). Since
-its creation, this library has grown beyond Boost.Python in many ways, leading
-to dramatically simpler binding code in many common situations.
+Python (2.7 or 3.5+, or PyPy) and the C++ standard library. This compact
+implementation was possible thanks to some of the new C++11 language features
+(specifically: tuples, lambda functions and variadic templates). Since its
+creation, this library has grown beyond Boost.Python in many ways, leading to
+dramatically simpler binding code in many common situations.
 
 Tutorial and reference documentation is provided at
-[http://pybind11.readthedocs.org/en/master](http://pybind11.readthedocs.org/en/master).
-A PDF version of the manual is available
-[here](https://media.readthedocs.org/pdf/pybind11/master/pybind11.pdf).
+[pybind11.readthedocs.org][].  A PDF version of the manual is available
+[here][docs-pdf].
 
 ## Core features
-pybind11 can map the following core C++ features to Python
+pybind11 can map the following core C++ features to Python:
 
 - Functions accepting and returning custom data structures per value, reference, or pointer
 - Instance methods and static methods
@@ -51,15 +49,15 @@ pybind11 can map the following core C++ features to Python
 - Custom operators
 - Single and multiple inheritance
 - STL data structures
-- Smart pointers with reference counting like ``std::shared_ptr``
+- Smart pointers with reference counting like `std::shared_ptr`
 - Internal references with correct reference counting
 - C++ classes with virtual (and pure virtual) methods can be extended in Python
 
 ## Goodies
 In addition to the core functionality, pybind11 provides some extra goodies:
 
-- Python 2.7, 3.x, and PyPy (PyPy2.7 >= 5.7) are supported with an
-  implementation-agnostic interface.
+- Python 2.7, 3.5+, and PyPy (tested on 7.3) are supported with an implementation-agnostic
+  interface.
 
 - It is possible to bind C++11 lambda functions with captured variables. The
   lambda capture data is stored inside the resulting Python function object.
@@ -83,10 +81,10 @@ In addition to the core functionality, pybind11 provides some extra goodies:
 - Binaries are generally smaller by a factor of at least 2 compared to
   equivalent bindings generated by Boost.Python. A recent pybind11 conversion
   of PyRosetta, an enormous Boost.Python binding project,
-  [reported](http://graylab.jhu.edu/RosettaCon2016/PyRosetta-4.pdf) a binary
-  size reduction of **5.4x** and compile time reduction by **5.8x**.
+  [reported][pyrosetta-report] a binary size reduction of **5.4x** and compile
+  time reduction by **5.8x**.
 
-- Function signatures are precomputed at compile time (using ``constexpr``),
+- Function signatures are precomputed at compile time (using `constexpr`),
   leading to smaller binaries.
 
 - With little extra effort, C++ types can be pickled and unpickled similar to
@@ -97,7 +95,8 @@ In addition to the core functionality, pybind11 provides some extra goodies:
 1. Clang/LLVM 3.3 or newer (for Apple Xcode's clang, this is 5.0.0 or newer)
 2. GCC 4.8 or newer
 3. Microsoft Visual Studio 2015 Update 3 or newer
-4. Intel C++ compiler 17 or newer (16 with pybind11 v2.0 and 15 with pybind11 v2.0 and a [workaround](https://github.com/pybind/pybind11/issues/276))
+4. Intel C++ compiler 17 or newer (16 with pybind11 v2.0 and 15 with pybind11
+   v2.0 and a [workaround][intel-15-workaround])
 5. Cygwin/GCC (tested on 2.5.1)
 
 ## About
@@ -122,8 +121,23 @@ Henry Schreiner,
 Ivan Smirnov, and
 Patrick Stewart.
 
+### Contributing
+
+See the [contributing guide][] for information on building and contributing to
+pybind11.
+
+
 ### License
 
 pybind11 is provided under a BSD-style license that can be found in the
-``LICENSE`` file. By using, distributing, or contributing to this project,
+[`LICENSE`][] file. By using, distributing, or contributing to this project,
 you agree to the terms and conditions of this license.
+
+
+[pybind11.readthedocs.org]: http://pybind11.readthedocs.org/en/master
+[docs-pdf]: https://media.readthedocs.org/pdf/pybind11/master/pybind11.pdf
+[Boost.Python]: http://www.boost.org/doc/libs/1_58_0/libs/python/doc/
+[pyrosetta-report]: http://graylab.jhu.edu/RosettaCon2016/PyRosetta-4.pdf
+[contributing guide]:  https://github.com/pybind/pybind11/blob/master/CONTRIBUTING.md
+[`LICENSE`]: https://github.com/pybind/pybind11/blob/master/LICENSE
+[intel-15-workaround]: https://github.com/pybind/pybind11/issues/276