Reference active forks in CONTRIBUTING.md

Author: ianmacgillivray

CONTRIBUTING.md

@@ -2,7 +2,8 @@
 
 We encourage you to fork the repository; this repository does not currently
 accept patches or contributions. See README.md for rules governing
-the Blocks trademark.
+the Blocks trademark. See https://github.com/googlevr/blocks/network for a
+list of active forks.
 
 Should our contribution policy ever change, contributors will need to
 follow a few small guidelines.
@@ -30,4 +31,4 @@ information on using pull requests.
 ## Community Guidelines
 
 This project follows
-[Google's Open Source Community Guidelines](https://opensource.google/conduct/).
+[Google's Open Source Community Guidelines](https://opensource.google/conduct/).