Add CONTRIBUTING.md
Add CONTRIBUTING.md

This is almost an exact copy of the ReactiveCocoa contributing guide.
Only tweak was to link to the swift style guide instead of obj-c.

ikesyo/Result,ratkins/Result,natestedman/Result,NachoSoto/Result,ikesyo/Result,ratkins/Result,NachoSoto/Result,NachoSoto/Result,antitypical/Result,ikesyo/Result,natestedman/Result,natestedman/Result,antitypical/Result,ratkins/Result,antitypical/Result