update README.md to reflect gitlab flow

@@ -7,10 +7,7 @@ as well as ebuilds for my own projects, as far as I have them.
 
 To add this overlay:
 
-    ~> HOST=gitlab.weird-web-workers.org
-    ~> BASE_URL=https://${HOST}/ghopp/ghopp-overlay
-    ~> XML=${BASE_URL}/raw/master/overlay.xml
-    ~> layman -o ${XML} -f -a ghopp-overlay
+    ~> layman -o http://gitlab.weird-web-workers.org/ghopp/ghopp-overlay/raw/production/overlay.xml -f -a ghopp
     ~> layman-updater -R
     ~> layman -a ghopp