Add reference to the mailing list to the README.

Change-Id: Ifb4c40ff087cd00242ecd33306b0ad1d8dcc411d
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/5920
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

README.md

diff --git a/README.md b/README.md
index 8ac7837..a8d9fa3 100644
--- a/README.md
+++ b/README.md
@@ -1,9 +1,13 @@
 # GN
 
 GN is a meta-build system that generates build files for
-[Ninja](https://ninja-build.org). There is documentation in
-[docs/](https://gn.googlesource.com/gn/+/master/docs/) and
-[a presentation on it](https://docs.google.com/presentation/d/15Zwb53JcncHfEwHpnG_PoIbbzQ3GQi_cpujYwbpcbZo/edit?usp=sharing).
+[Ninja](https://ninja-build.org).
+
+Related resources:
+
+  * Documentation in [docs/](https://gn.googlesource.com/gn/+/master/docs/).
+  * An introductory [presentation](https://docs.google.com/presentation/d/15Zwb53JcncHfEwHpnG_PoIbbzQ3GQi_cpujYwbpcbZo/edit?usp=sharing).
+  * The [mailing list](https://groups.google.com/a/chromium.org/forum/#!forum/gn-dev).
 
 ## Getting a binary