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
