Merge "Add xmlsec1 dependency comments"

doc/source/extensions/federation.rst

@@ -56,3 +56,10 @@ To enable the federation extension:
    by default, it must be installed manually. For example::
 
         pip install --upgrade $(grep pysaml2 test-requirements.txt)
+
+   Also, the `xmlsec1` command line tool is needed to sign the SAML assertions
+   generated by the Keystone Identity Provider:
+
+   .. code-block:: bash
+
+        $ apt-get install xmlsec1