Adds new 'Contribute' entry to README
Adds new 'Contribute' entry to README

Information on how to contribute with the project has been added to the
README file. It’s some basic information around the recommended usage
of git and GitHub.

http2d/core,alobbs/libhpack,alobbs/libhpack,http2d/core,http2d/core,alobbs/libhpack,http2d/core,alobbs/libhpack