Documentation changes to reflect new plugin assetlink stuff

- updated old theme assetlink section to reflect new location of
   ./bin/gmg assetlink and removed comment about the plugin command
   being temporary.
 - Added a new section to the standard config file on where to put the
   plugin_static section
 - Added release notes about said section

This commit sponsored by Thomas Webber.  Thanks, Dad!

docs/source/siteadmin/deploying.rst

@@ -327,6 +327,11 @@ this ``nginx.conf`` file should be modeled on the following::
         alias /srv/mediagoblin.example.org/mediagoblin/user_dev/theme_static/;
      }
 
+     # Plugin static files (usually symlinked in)
+     location /plugin_static/ {
+        alias /srv/mediagoblin.example.org/mediagoblin/user_dev/plugin_static/;
+     }
+
      # Mounting MediaGoblin itself via FastCGI.
      location / {
         fastcgi_pass 127.0.0.1:26543;