Update README with custom themes

Mention how to use custom themes by placing them in custom/themes.

README.markdown

@@ -37,6 +37,9 @@ If you would like to override the functionality of a plugin distributed with oh-
 create a plugin of the same name in the `custom/plugins/` directory and it will be loaded
 instead of the one shipped with oh-my-fish.
 
+If you would to use your custom theme, move it with the same name in the `custom/themes/` directory
+and it will override the original theme in `themes/`.
+
 If you just want to override any of the default behavior or add some environment variables,
 just add a new file (ending in .load) into the `custom/` directory.