Update custom_plugins.rst

add a description to show a foreign key relation in the edit mode of a plug in. May be i am wrong but i didn't found a other way to show the relation in the inline admin.
Merge branch 'develop' of https://github.com/kater169/django-cms into kater169-develop
Fix formatting issues
Merge pull request #3197 from yakky/kater169-develop

Update custom_plugins.rst