CONTRIBUTING: Split development notes into their own file

The previous README contained notes for example-lesson developers
(e.g. Git workflow notes) as well as notes for lesson consumers
(e.g. _config_template.yml copying).  This commit splits the former
into GitHub's preferred file [1].  That ensures the license
information, code of conduct, and other development-critical notes are
highlighted and easily accessible to anyone filing and issue or
pull-request against this repository.

[1]: https://github.com/blog/1184-contributing-guidelines