Add contribution guidelines & code of conduct

[activescott]

Lets use this issue to discuss contribution guidelines and a code of conduct. Maybe we can link to examples from other open source projects that the community and maintainers like to start putting these together.

[activescott]

Effective, short enough that someone will actually read it: https://developer.twitter.com/en/community/code-of-conduct

[freeatnet]

@activescott Thanks for bringing it up! I like the code of conduct linked, even though it seems to be written for events rather than communities. Could you put together a similar code of conduct that we can use?

As far as contribution guidelines: I'd like to add them in the new repos, since we're hoping to sunset this part of the system sooner rather than later.

[activescott]

Great starting point: http://todogroup.org/opencodeofconduct/