Update README.rst

TEXT_ADDITIONAL_TAGS was not enough to make this work for me, when HTML is entered into the editor manually.

The editor sanitizes tags before it gets to the server, so the tags in question are treated as text-only.

This is how I fixed it.
How about the phrasing to something like: "you may need to add the following parameters to ``CKEDITOR_SETTINGS``::" (and then just adding the two parameters as bullet point or something like that)
This will lead to less error when copy-pasting (novice users may think that they have to replace the existing ``CKEDITOR_SETTINGS`` with the dictionary provided below