Adding more doc

Author: moufmouf

doc/magic_join.md

@@ -65,3 +65,17 @@ Now that you understand the concept, you should understand the power and the lim
 
 <div class="alert alert-warning">MagicJoin is meant to be used on the 80% of the cases where writing joins is trivial
 and boring. If you have complex joins, do not try to use MagicJoin. Go back to pure SQL instead.</div>
+
+###With great power comes great responsibility
+
+MagicJoin is a powerful feature because it can **guess** what you want to do. Please be wise while using it.
+A change in your model could modify the shortest path computed by MagicJoin and therefore the returned SQL query.
+
+If you database model is complex enough and is rapidly changing, generated queries could suddenly change in 
+your application.
+
+Please consider one of these 2 options:
+
+- write unit tests (you have unit tests, don't you?)
+- use MagicJoin in development mode only. While developing, dump the SQL generated by MagicJoin and put it back
+  in your code.