Remind people about the new template in the upgrade notes

UPGRADE.rst

@@ -75,6 +75,27 @@ for example:
      wget https://packages.matrix.org/debian/pool/main/m/matrix-synapse-py3/matrix-synapse-py3_1.3.0+stretch1_amd64.deb
      dpkg -i matrix-synapse-py3_1.3.0+stretch1_amd64.deb
 
+Upgrading to v1.19.0
+====================
+
+New HTML templates
+------------------
+
+A new HTML template,
+`password_reset_confirmation.html <https://github.com/matrix-org/synapse/blob/develop/synapse/res/templates/password_reset_confirmation.html>`_,
+has been added to the ``synapse/res/templates`` directory. If you are using a
+custom template directory, you will need to copy the template over and modify
+it if necessary.
+
+This page will appear to the user after clicking a password reset link that has
+been emailed to them.
+
+To complete password reset, the page must include a way to make a `POST`
+request to
+``/_synapse/client/account/v1/password_reset/{medium}/submit_token_confirm``
+with the query parameters from the original link. See the file itself for more
+details.
+
 Upgrading to v1.18.0
 ====================