In the docs, mention that FEINCMS_RICHTEXT_INIT_TEMPLATE needs to be set if using TinyMCE 4.x

When using TinyMCE 4.x the user has to set FEINCMS_RICHTEXT_INIT_TEMPLATE to the init_tinymce4.html template for the rich text content to work correctly.
Merge pull request #475 from adsworth/master

In the docs, mention that FEINCMS_RICHTEXT_INIT_TEMPLATE needs to be set...