Add notes about the CLA and gn-dev@ to the README.

Change-Id: Ic58326bbb79c7a25aca2007b543a332e6dca951a
Reviewed-on: https://gn-review.googlesource.com/2380
Reviewed-by: Petr Hosek <phosek@google.com>
Commit-Queue: Petr Hosek <phosek@google.com>

README.md

diff --git a/README.md b/README.md
index 7027121..0f30089 100644
--- a/README.md
+++ b/README.md
@@ -39,4 +39,13 @@
 which will add the new changes to the existing code review, rather than creating
 a new one.
 
-TODO: AUTHORS, CLA, etc.
+We ask that all contributors
+[sign Google's Contributor License Agreement](https://cla.developers.google.com/)
+(either individual or corporate as appropriate, select 'any other Google
+project').
+
+## Community
+
+You may ask questions and follow along w/ GN's development on Chromium's
+[gn-dev@](https://groups.google.com/a/chromium.org/forum/#!forum/gn-dev)
+Google Group.