Updated readme.html to point to nginx config in wiki. Removed outdated links.

The Yourls wiki has [a page](https://github.com/YOURLS/YOURLS/wiki/Nginx-configuration) about configuring nginx: The file `readme.html` should point to it. (Or maybe, add short link http://yourls.org/nginx pointing to the wiki?) 

The domain `http://rml.me` is currently for sale and should be removed from the server recommendation list.

The page describing the Seesmic plugin seems not available anymore.