Added list of BBcodes that work and don't

README.md

@@ -24,6 +24,40 @@ _**Note**: It has "zen" on the beginning of the name just to make the add-on loa
 
 If no text is selected in the editor a Postbox will be appeneded to the text area with "..." for content. Replace the "..." with your text, etc.
 
+While Postbox allows more content than the Facebook version (which is text only), there are limitations due to how Friendica parses BBcode.
+
+**It is STRONGLY recommended you only use Postboxes with text and emoji.**
+
+BBcodes you CANNOT put inside a Postbox:
+
+* [class] (which means you can’t nest Postboxes)
+* [hr]
+* [h1],[h2],[h3], etc…
+* [table],[tr],[th],[td]
+* [list],[ul],[ol]
+* [abstract]
+* [spoiler]
+* [map]
+* [code]
+
+BBcodes that do not work as intended inside a Postbox:
+
+* [pre]
+* [noparse]
+* [nobb]
+
+The text will show but will be styled and centered.
+
+BBcodes that **DO WORK** inside a Postbox:
+* [url]
+* [img]
+* [audio]
+* [video]
+
+And any plain text, including emoji
+
+If you are using Markdown formatting what you can and can’t put in a Postbox is similar, with the exception that (because of how Markdown is parsed into BBcode) you can’t have both a URL and an image in the same Postbox. You *can* however put inline `code` in a Postbox with Markdown where BBcode cannot.
+
 ## Known Issues
 
 - Postboxes are not shown on other platforms when your post is shared.