Make docs more helpful for end-users

I'd like to revamp the README and contributing docs to focus more on GitHub users since that seems to be the majority of the people that interact with this project.

- [x] Remove docs on adding new markup formats. This rarely happens.
- [ ] Explain the basics of how GitHub renders docs (e.g. highlighting the html-pipeline) so people have an idea where to report issues
- [ ] Point users to the upstream projects for reporting bugs in the various markup formats.
- [ ] Explicitly call out what this library does _not_ do (e.g. CSS, syntax highlighting, markdown extensions)

/cc @github/user-content