Configurable SSH access to GitHub

Change-Id: I2ad5b17991d25ec7492d931c069cbcf314dba804
diff --git a/doc/source/connections.rst b/doc/source/connections.rst
index 521e4ed..7b302e9 100644
--- a/doc/source/connections.rst
+++ b/doc/source/connections.rst
@@ -83,6 +83,10 @@
   See `Securing your webhooks
   <https://developer.github.com/webhooks/securing/>`_.
 
+**sshkey**
+  Path to SSH key to use when cloning github repositories.
+  ``sshkey=/home/zuul/.ssh/id_rsa``
+
 SMTP
 ----