added a CONTRIBUTING.md file for the GitHub issue tracker

Loosely based on the template from the 'drush' project

CONTRIBUTING.md

+Thank you for your interest in this project! Please refer to the following
+sections on how to contribute code and bug reports.
+
+### Reporting bugs
+
+At the moment, this project is run in the spare time of a single person
+([Wenzel Jakob](http://rgl.epfl.ch/people/wjakob)) with very limited resources
+for issue tracker tickets. Thus, before submitting a question or bug report,
+please take a moment of your time and ensure that your issue isn't already
+discussed in the project documentation provided at
+[http://pybind11.readthedocs.org/en/latest](http://pybind11.readthedocs.org/en/latest).
+
+Assuming that you have identified a previously unknown problem or an important
+question, it's essential that you submit a self-contained and minimal piece of
+code that reproduces the problem. In other words: no external dependencies,
+isolate the function(s) that cause breakage, submit matched and complete C++
+and Python snippets that can be easily compiled and run on my end.
+
+## Pull requests
+Contributions are submitted, reviewed, and accepted using Github pull requests.
+Please refer to [this
+article](https://help.github.com/articles/using-pull-requests) for details and
+adhere to the following rules to make the process as smooth as possible:
+
+* Make a new branch for every feature you're working on.
+* Make small and clean pull requests that are easy to review but make sure they
+  do add value by themselves.
+* Add tests for any new functionality and run the test suite (``make test``) to
+  ensure that no existing features break.
+* This project has a strong focus on providing general solutions using a
+  minimal amount of code, thus small pull requests are greatly preferred.
+
+### 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, you
+agree to the terms and conditions of this license.