docs for Push Rules updated with Custom Server Side Hooks link

doc/push_rules/push_rules.md

@@ -49,6 +49,13 @@ branch names globally in Push Rules, you can now sleep without the anxiety
 of your developers' mistakes. Every branch that doesn't match your push rule
 will get rejected.
 
+### Custom Push Rules **(CORE ONLY)**
+
+It's possible to create custom push rules rather than the push rules available in
+**Admin area > Push Rules** by using more advanced server-side Git hooks.
+
+See [custom server-side Git hooks](../administration/custom_hooks.md) for more information.
+
 ## Enabling push rules
 
 NOTE: **Note:**