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

CONTRIBUTING.md

@@ -0,0 +1,10 @@
+# Contributing to LLVM
+
+Thank you for your interest in contributing to LLVM! There are many ways to
+contribute, and we appreciate all contributions.
+
+To get started with contributing, please take a look at the
+[Contributing to LLVM](https://llvm.org/docs/Contributing.html) guide. It
+describes how to get involved, raise issues and submit patches. Please note
+that at the moment the LLVM project does not use either Github pull requests
+or Github issues.

README.md

@@ -4,6 +4,11 @@ This directory and its subdirectories contain source code for LLVM,
 a toolkit for the construction of highly optimized compilers,
 optimizers, and runtime environments.
 
+The README briefly describes how to get started with building LLVM.
+For more information on how to contribute to the LLVM project, please
+take a look at the
+[Contributing to LLVM](https://llvm.org/docs/Contributing.html) guide.
+
 ## Getting Started with the LLVM System
 
 Taken from https://llvm.org/docs/GettingStarted.html.