Make clear that rack-rewrite uses only PATH_INFO, not SCRIPT_NAME

[ameuret]

Apache, Unicorn, nGinX, Webrick, Mongrel2 AND the myriad of Rack adapters don't really agree on the way the request URI should be split and which ENV variable drives it. It leads to much frustration as rules seem to stop working when switching web servers or adapters (read dev vs. prod).

A simple "Heads up!" in the README pointing to these issues (and the involvement of RACK_RELATIVE_URL_ROOT or RAILS_RELATIVE_URL_ROOT) will help a lot of people dealing with this.