Document how to contribute

[mikebentley15]

Many projects have documented what others can do to contribute to a project. FLiT does not yet have this.

• As of the addressing of issue #182, we will have a continuous integration system. This should be mentioned.
• Part of this guide should address coding standards and what to do before making a pull request (such as running unit tests, running pylint on any modified files, and running python doctest on any modified python files)
• Every pull request should have documentation and tests, or a description of why they do not apply.
• What other kinds of things people can work on (translation, internationalization, issues, pull requests, voting for feature requests, documentation, etc)
• Talk about how to donate money or why we do not accept money (maybe something to talk about)

So far there is not a huge following of FLiT, so not too many contributors. But it is a good tool that I hope will get at least a little recognition soon, especially after the next paper. When that happens, we will want documentation discussing how to contribute back.