DOC: add that it is cross platform

PR-URL: #291
Credit: @developingAlex
Close: #291
Reviewed-by: @isaacs

README.md

@@ -1,4 +1,5 @@
-The [UNIX command](<http://en.wikipedia.org/wiki/Rm_(Unix)>) `rm -rf` for node.
+The [UNIX command](<http://en.wikipedia.org/wiki/Rm_(Unix)>) `rm -rf` for node
+in a cross-platform implementation.
 
 Install with `npm install rimraf`.