docs: add additional description of the middleware

Author: dougwilson

README.md

@@ -27,7 +27,9 @@ var compression = require('compression')
 
 ### compression([options])
 
-Returns the compression middleware using the given `options`.
+Returns the compression middleware using the given `options`. The middleware
+will attempt to compress response bodies for all request that traverse through
+the middleware, based on the given `options`.
 
 #### Options