doc: point to the Handbook repo

doc/README.md

@@ -1,3 +1,3 @@
 # Documentation
 
-The Jitsi documentation has been moved to [The Handbook](https://jitsi.github.io/handbook/).
+The Jitsi documentation has been moved to [The Handbook](https://jitsi.github.io/handbook/). The repo is https://github.com/jitsi/handbook.