doc: http: expand request.headers documentation

doc/api/http.markdown

@@ -265,7 +265,16 @@ you can use the `require('querystring').parse` function, or pass
 
 ### request.headers
 
-Read only.
+Read only map of header names and values. Header names are lower-cased.
+Example:
+
+    // Prints something like:
+    //
+    // { 'user-agent': 'curl/7.22.0',
+    //   host: '127.0.0.1:8000',
+    //   accept: '*/*' }
+    console.log(request.headers);
+
 
 ### request.trailers