Add note about rich editor for original fields in README.md

I have missed that rich editor will not be automatically installed for all admin text areas.

I haven't seen it even at a second glance, reading README very carefully.

So this is my proposal how to help others like me to see at first that original Sylius admin fields have to be extended manually.

README.md

@@ -72,6 +72,8 @@ then we will convert it for you as an HTML Element which contains… HTML.
 
 This way you will be able to use our plugin right away without risking any data lost!
 
+To make it clear: to have rich editor for original admin fields, like _Product description_, _Taxon desription_ etc. you have to update each form type by an extension as the example shows above.
+
 ### Call twig render
 
 To display the content of the rich editor field you must call the twig filter: