Add a CONTRIBUTING guide.
Add a CONTRIBUTING guide.

GitHub will show this when someone's contributing. Hopefully it will
lessen the number of pull requests.

brennie/reviewboard,bkochendorfer/reviewboard,chipx86/reviewboard,1tush/reviewboard,KnowNo/reviewboard,beol/reviewboard,1tush/reviewboard,chipx86/reviewboard,bkochendorfer/reviewboard,beol/reviewboard,custode/reviewboard,KnowNo/reviewboard,davidt/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,bkochendorfer/reviewboard,1tush/reviewboard,reviewboard/reviewboard,1tush/reviewboard,davidt/reviewboard,davidt/reviewboard,bkochendorfer/reviewboard,sgallagher/reviewboard,custode/reviewboard,1tush/reviewboard,beol/reviewboard,sgallagher/reviewboard,davidt/reviewboard,1tush/reviewboard,brennie/reviewboard,chipx86/reviewboard,custode/reviewboard,KnowNo/reviewboard,KnowNo/reviewboard,sgallagher/reviewboard,beol/reviewboard,custode/reviewboard,brennie/reviewboard,reviewboard/reviewboard,1tush/reviewboard,1tush/reviewboard,brennie/reviewboard,1tush/reviewboard,sgallagher/reviewboard