Added instruction to join the llvm discourse and discord group.

Added instruction and link to join the llvm discord and discourse group
in the CONTRIBUTING.md files

Reviewed By: keith

Differential Revision: https://reviews.llvm.org/D146877

CONTRIBUTING.md

@@ -7,3 +7,12 @@ 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 GitHub pull requests.
+
+## Getting in touch
+
+Join the [LLVM Discourse forums](https://discourse.llvm.org/), [Discord
+chat](https://discord.gg/xS7Z362), or #llvm IRC channel on
+[OFTC](https://oftc.net/).
+
+The LLVM project has adopted a [code of conduct](https://llvm.org/docs/CodeOfConduct.html) for
+participants to all modes of communication within the project.