README.md: Add a link to the mailing list.

BUG=

Change-Id: Ieab1e83ce5126881421f0f7a456476db3fde2a0a
Reviewed-on: https://dawn-review.googlesource.com/c/3242
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/README.md b/README.md
index e9def8c..d262b40 100644
--- a/README.md
+++ b/README.md
@@ -68,6 +68,7 @@
 Overall try to use the same style and convention as code around your change.
 
 If you find issues with Dawn, please feel free to report them on the [bug tracker](https://bugs.chromium.org/p/dawn/issues/entry).
+For other discussions, please post to [Dawn's mailing list](https://groups.google.com/forum/#!members/dawn-graphics).
 
 ## License