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

Reviewed and approved by chandlerc.

As GitHub is the canonical LLVM repository now, embrace GitHub's way of displaying basic build instructions in the top-level readme.md.

Per llvm.org: "The name "LLVM" itself is not an acronym; it is the full
name of the project."

llvm-svn: 344766

monorepo.

llvm-svn: 316185