Add social link to @GerritReview Twitter

Change-Id: I3d838cf66d44f2e74c3241bd856a60917184a5fb
diff --git a/index.md b/index.md
index 8da622a..1307ab9 100644
--- a/index.md
+++ b/index.md
@@ -81,6 +81,8 @@
 
 [![Google+](https://ssl.gstatic.com/images/icons/gplus-32.png)](https://plus.google.com/communities/111271594706618791655)
 
+[![Twitter](https://www.gerritforge.com/images/Twitter_Logo_Blue.png)](https://twitter.com/gerritreview)
+
 ### IRC
 
 [freenode](https://freenode.net/) #gerrit