[Docs] Add Contributing page.

This new page acts as an entry point for (new) contributors to LLVM and
provides information about

* What to contribute
* How to submit a patch
* Where to start to learn more about LLVM's architecture and internals.

This version of the page duplicates some information from the
DeveloperPolicy and Phabricator pages. Subsequent changes should work
towards moving information for new developers to this page, where it
makes sense.

Reviewers: reames, probinson, kristof.beyls, silvas, rengolin, asb

Reviewed By: silvas

Differential Revision: https://reviews.llvm.org/D41665

llvm-svn: 321804