Add Project source code URL comment to matrix-sliding-sync role

roles/custom/matrix-sliding-sync/defaults/main.yml

@@ -1,6 +1,7 @@
 ---
 
 # Sliding Sync Proxy is an implementation of MSC3575 for the new sliding sync
+# Project source code URL: https://github.com/matrix-org/sliding-sync
 
 matrix_sliding_sync_enabled: true