Document how to set clipboard-write permission

docs/publishers/embedding.rst

@@ -44,3 +44,29 @@ document and have the same origin.
 
 The client will watch for new iframes being added to the document and will
 automatically enable annotation for them.
+
+
+Clipboard permissions when loaded in an iframe
+----------------------------------------------
+
+There is a few places in the sidebar where the user can copy content to the
+clipboard:
+
+* The link, when sharing an annotation.
+* The sidebar's version.
+* The content, when exporting annotations to any format.
+* Etc.
+
+This is done through the browser's
+`Clipboard API <https://developer.mozilla.org/en-US/docs/Web/API/Clipboard_API>`_,
+which requires allowing the ``clipboard-write`` permission.
+
+When loading the sidebar in the top-level document, this should work out of
+the box, but if you load Hypothesis inside an iframe, remember adding the
+allow attribute with the right permissions.
+
+.. code-block:: html
+
+   <iframe allow="clipboard-write">
+     <!-- Hypothesis is loaded here -->
+   </iframe>