DOC: Explain how our shared_docs system works
Enhance https://github.com/pandas-dev/pandas/blob/master/doc/source/contributing.rst with a discussion of how our `_shared_docs` system works.

- Why we do it (reduce duplicate docstrings, while still having some class-specific stuff)
- How it's done (Appender, _shared_docs dict, _shared_doc_kwargs)
- How to substitute values