Doc clarification on GitHub repos

GitHub lists and recommends HTTPS addresses for repos hosted there, not the SSH style needed by Weblate. Easy to miss this without thinking and proceeding to pull out hair for two days while wondering why Weblate cannot get the user to use for pushing :-)

docs/admin/projects.rst

@@ -231,6 +231,9 @@ You also need to verify SSH host keys of servers you are going to access.
 You can generate or display key currently used by Weblate in the admin
 interface (follow :guilabel:`SSH keys` link on main admin page).
 
+If you are trying to connect to a GitHub repository be sure to use the SSH
+address, not the default HTTPS address. It should start with 'git@github.com'
+
 .. note::
 
     The keys need to be without password to make it work, so be sure they are