Add link to manual source from manual index.

index.md

@@ -2,3 +2,5 @@ This is the manual for [VCV Rack](https://vcvrack.com/), the open-source Eurorac
 
 For the manual of a specific module, find it on the [VCV Library](https://library.vcvrack.com/) and click the "User manual" link.
 Or, right-click the module in VCV Rack and choose "Plugin > Manual".
+
+The source of this manual is available at https://github.com/VCVRack/manual.