[exim-dev] CONTRIBUTING text file

I wrote up a CONTRIBUTING file for Exim, pushed to my repo:

http://git.exim.org/users/pdp/exim.git/blob/03f9df9f25b65d7bfac0f1b4965fe7c38f952f24:/src/CONTRIBUTING

Any comments or objections?

(It assumes the new website)

-Phil