Added README

README.md

+OAuth2 Proxy for Single-Sign-On with Nginx
+==========================================
+
+This application implements a generic OAuth2 client for easy integration of
+NGINX into an OAuth2-based Single-Sign-On system. It is bound to a single
+OAuth2 server (urls defined in config) and -- apart from a session cookie --
+stateless. OAuth2 client id, secret and redirect URI are passed from the
+integrating NGINX to the proxy in HTTP headers.
+
+See testapp for an example setup.