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

Because we're going to be storing maintainer information locally in
the README.md [1].

[1]: https://github.com/swcarpentry/lesson-template/issues/195#issuecomment-89643048

Downstream lesson maintainers are supposed to adapt this to point at
their repository's issues [1], so use a reference name that works for
both the lesson-template and downstream repositories.  That way they
only have to update the referenced URL and can leave the reference
name alone.

[1]: https://github.com/swcarpentry/lesson-template/issues/195#issuecomment-78310923

1.  Removed YAML header.
2.  Changed link to refer to `LICENSE.html` instead of `LICENSE.md`.

Leaving the reference to the lesson-template repo alone - lesson authors will be expected to edit this (and instructions to do that will go in the docs in the gh-pages branch).