Add upgrade guide to the documentation

[skip ci]

docs/changelog.rst

@@ -60,7 +60,8 @@ v2.2.0 (Not yet released)
 
 * pybind11's headers and build system now more strictly enforce hidden symbol
   visibility for extension modules. This should be seamless for most users,
-  but see the upgrade guide if you use a custom build system.
+  but see the :doc:`upgrade` if you use a custom build system.
+  `#995 <https://github.com/pybind/pybind11/pull/995>`_.
 
 * Support for ``py::module_local`` types which allow multiple modules to
   export the same C++ types without conflicts. This is useful for opaque

docs/index.rst

@@ -14,6 +14,7 @@ pybind11 --- Seamless operability between C++11 and Python
 
    intro
    changelog
+   upgrade
 
 .. toctree::
    :caption: The Basics