Add basic contribution documentation

[stkent]

I am interested in contributing to this project, but I'm having trouble getting it up and running on my local system. (I'm much more familiar with Gradle than with Maven). It would be great if the README included setup instructions that would allow me to build, deploy locally, run tests, etc. 😃

[stkent]

Hi! Looking for a response to this issue today; otherwise I'll need to fork and make breaking changes to move forward.

[thombergs]

Sorry, the notification about this issue escaped me. I'll keep this issue open to re-integrate your fork into the master at some point.

[stkent]

No worries. My fork includes some major functionality changes, such as focusing entirely on GitHub's diff format as opposed to handling the more general unified diff format. Re-integration is probably not feasible; however, it should be quick for you to browse the code in my fork and see if anything makes sense as a patch for this repo. Let me know if you choose to do this and have any questions about the changes I made. Cheers!