Add note on openrc and zsh

Add a note about running openrc through bash for import into zsh, as
shown by Chmouel Boudjnah in I5b6c8cfedcdd36efb4cbc91831501ee5c9c3b1d1

Change-Id: I5f1c9fc3fdc045cf6fb69af13f6264a81bf5f763
Closes-Bug: #1460656
diff --git a/doc/source/faq.rst b/doc/source/faq.rst
index 87f8469..b09d386 100644
--- a/doc/source/faq.rst
+++ b/doc/source/faq.rst
@@ -75,6 +75,21 @@
 Some people have success with bash 4 installed via homebrew to keep
 running tests on OS/X.
 
+Can I at least source ``openrc`` with ``zsh``?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+People have reported success with a special function to run ``openrc``
+through bash for this
+
+.. code-block:: bash
+
+   function sourceopenrc {
+       pushd ~/devstack >/dev/null
+       eval $(bash -c ". openrc $1 $2;env|sed -n '/OS_/ { s/^/export /;p}'")
+       popd >/dev/null
+   }
+
+
 Operation and Configuration
 ===========================