Create CONTRIBUTING.md
Create CONTRIBUTING.md

I added the following to it:

- Bug reporting
- Feature requests
- Contributing guide
- Style guide
tobscure/mithril.js,pygy/mithril.js,pygy/mithril.js,lhorie/mithril.js,impinball/mithril.js,mrvicadai/mithril.js,MithrilJS/mithril.js,barneycarroll/mithril.js,impinball/mithril.js,lhorie/mithril.js,barneycarroll/mithril.js,mrvicadai/mithril.js,tobscure/mithril.js,MithrilJS/mithril.js,tivac/mithril.js,maxlang/mithril.js,maxlang/mithril.js,tivac/mithril.js