Add support files

[bombaywalla]

This is my first cut at adding community support files. In particular, I've added

• CONTRIBUTING.md about how to make contributions to this repo.
• CHANGELOG.md to document changes.
• RELEASE.md to document how to make releases.

I've tried to retrieve the changes made in the past and incorporate them into the CHANGELOG.md, but they may be incomplete.

My understanding of Github releases is rudimentary, so please verify that what I have said is correct.

Any comments or suggestions for improvement are welcome.

I have not yet added a PR template, but wanted to get these published first, so that we can get some experiences with the release process.

[bombaywalla]

I am somewhat concerned that the release instructions are too heavyweight - in that they try to do a release via PRs. Perhaps there is a more expedient way? Feedback appreciated.

Also, not sure how the releases to clojars are done currently. I believe @yogthos has done the last few.

[bombaywalla]

I have pushed changes to this PR as a result of your comments. Please review and comment. Thanks!

[bombaywalla]

If you think the current version is okay, would you please merge this PR to master? Thanks.