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.
Dynamic template directory
Add tests for custom template dir
Add documentation
Improve template checking system
Improve tests checking actual template loading and for badly placed templates directory
Add support for site-enabled CMS_TEMPLATES_DIR
Switch to module-based templates dir configuration. This allow for easier translation of template names.
Update documentation
If JavaScript sends empty string as the language parameter when re-ordering, use language from the plugin
Fix #3189, get_plugins_for_page didn't resolve inherited template the right way
Fix formatting issues
Added PAGE_MENU_IDENTIFIER and HISTORY_MENU_IDENTIFIER to map the existing ADMIN_MENU_IDENTIFIER
target in the GET request params will be an empty string if creating a top-level page
Add mptt 0.6.1 to allowed version