Added installation instructions

There are number of oauth solutions for node.js and it is
helpful to provide the official installation instructions
in the Readme file.  Installation instructions can be a little
confusing for this module because the github repository name
is node-oauth and there is a different  node.js oauth
solution that isntalls via npm with the command node-oauth.
Since this solution installs with oauth official
instructions may be helpful to some developers.

Readme.md

@@ -7,6 +7,12 @@ Tested against Twitter (http://twitter.com), term.ie (http://term.ie/oauth/examp
 Also provides rudimentary OAuth2 support, tested against facebook, github, foursquare, google and Janrain.   For more complete usage examples please take a look at connect-auth (http://github.com/ciaranj/connect-auth)
 
 
+Installation
+============== 
+
+    $ npm install oauth
+
+
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