Add documentation about auth.bearer

README.md

@@ -136,6 +136,14 @@ request.get('http://some.server.com/', {
     'sendImmediately': false
   }
 });
+// or
+request.get('http://some.server.com/').auth(null, null, true, 'bearerToken');
+// or
+request.get('http://some.server.com/', {
+  'auth': {
+    'bearer': 'bearerToken'
+  }
+});
 ```
 
 If passed as an option, `auth` should be a hash containing values `user` || `username`, `pass` || `password`, and `sendImmediately` (optional).  The method form takes parameters `auth(username, password, sendImmediately)`.
@@ -144,6 +152,8 @@ If passed as an option, `auth` should be a hash containing values `user` || `use
 
 Digest authentication is supported, but it only works with `sendImmediately` set to `false`; otherwise `request` will send basic authentication on the initial request, which will probably cause the request to fail.
 
+Bearer authentication is supported, and is activated when the `bearer` value is available. The value may be either a `String` or a `Function` returning a `String`. Using a function to supply the bearer token is particularly useful if used in conjuction with `defaults` to allow a single function to supply the last known token at the time or sending a request or to compute one on the fly.
+
 ## OAuth Signing
 
 ```javascript