DOC: Add calendar invite info (#59)

Author: seanpmorgan

README.md

@@ -78,6 +78,7 @@ By participating, you are expected to uphold this code.
 ## Community
 * [Public Mailing List](https://groups.google.com/a/tensorflow.org/forum/#!forum/addons)
 * [SIG Monthly Meeting Notes](https://docs.google.com/document/d/1kxg5xIHWLY7EMdOJCdSGgaPu27a9YKpupUz2VTXqTJg)
+    * Join our mailing list and receive calendar invites to the meeting
 
 ## License
 [Apache License 2.0](LICENSE)