Add a tip about where to put the migration runner (#557)

I've encountered the problem solved by this commit while I was learning Distillery through this tutorial - I think that it'd be nice to have the place for `App.ReleaseTasks` mentioned in the docs, especially for beginners.

Author: macbem

docs/guides/running_migrations.md

@@ -101,6 +101,8 @@ defmodule MyApp.ReleaseTasks do
   end
 end
 ```
+!!! warning
+    Remember to put this file under `lib`, as it must be compiled with the rest of your application, otherwise the code will not be available in the release, and the migrate command will fail.
 
 ## Custom command