add /src support to nginx config

Now that Wordpress develop has been added - we should handle the fact that it is in the /src directory.

It might be nice if this could intelligently check for the presence of the `/build` directory too and use that when available - in case you want to test the complied version.... although maybe nobody would want that!

I have a client site that uses a `public` directory in the git repo root - and I have just added a similar line to the nginx config to support this