Add publisher documentation about annotating iframed content.

docs/publishers/embedding.rst

@@ -13,3 +13,31 @@ each page that you want to have the Hypothesis client on:
 .. code-block:: html
 
    <script src="https://hypothes.is/embed.js" async></script>
+
+
+Enabling annotation of iframed content
+--------------------------------------
+
+The simplest way to support annotation of iframed content is to add the
+above script tag to the document displayed in the iframe. This will display the
+sidebar in the iframe itself.
+
+Additionally Hypothesis has limited support for enabling annotation of iframed
+content while showing the sidebar in the top-level document. To use this:
+
+1. Add the above script tag to the top-level document
+
+2. Do not add the script tag to the iframed documents themselves, the client
+   will do this itself.
+
+3. Opt iframes into annotation by adding the "enable-annotation" attribute:
+
+.. code-block:: html
+
+   <iframe enable-annotation>
+   ...
+   </iframe>
+
+This method *only* works for iframes which are same-origin with the top-level
+document. The client will watch for new iframes being added to the document and
+will automatically enable annotation for them.