Add contributing info to CONTRIBUTING.md and README.md

As discussed on llvm-dev [1], this patch adds a brief CONTRIBUTING.md to
the top-level of the repo, with a pointer to Contributing.html. This
should make it easier to discover the contributing information and also
be highlighted in the Github UI.

It also updates README.md to link to Contributing.html.

[1] http://lists.llvm.org/pipermail/llvm-dev/2019-November/137141.html

Reviewers: tonic, rnk, jhenderson, meikeb

Reviewed By: rnk, jhenderson, meikeb

Differential Revision: https://reviews.llvm.org/D70743