adding docu to :matching option

Author: Peter Schrammel

README.rdoc

@@ -34,7 +34,7 @@ revers_proxy_options set global options for all reverse proxies. Available optio
   * :preserve_proxy 
   * :username username for basic auth
   * :password password for basic auth
-  
+  * :matching is a global only option, if set to :first the first matched url will be requested (no ambigous error). Default: :all.
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