Add a bit of ssh universal configuration.

This is intended to work with the `ssh-mgmt` scripts.  Maybe its
better to move this there?  Maybe not?

startup_hooks/install.sh

@@ -72,3 +72,6 @@ echo "  signingKey = \"`hostname` Git\"" >> ~/.gitconfig
 echo "" >> ~/.gitconfig
 echo "[commit]" >> ~/.gitconfig
 echo "  gpgSign = true" >> ~/.gitconfig
+
+echo "Do not forget to add the following line at the top of your ssh config:"
+echo "Include ${CSHENV_LOCATION}/ssh/config"