Contributing.txt: what are your working on?

I think this file should make sure that new contributors don't make the mistake of not telling other devs what they are working on and consequently wasting their free time/ another developer's free time by working on the same issue.

I've also added "git diff --check" to the "Review your changes" instructions.
Merge pull request #366 from J-Shep/contributingsay_what_your_working_on

Contributing.txt: what are you working on?