docs(api): add links to upgrade & request

Author: ronag

README.md

@@ -87,6 +87,8 @@ server.on('request', (req, res) => {
 - `options`: see [Options](#options)
 - `onProxyError(err)`: called on error
 
+See [`request`](https://nodejs.org/api/http.html#http_event_request)
+
 #### ws (req, socket, head, options, onProxyError)
 
 - `req`: [`http.IncomingMessage`](https://nodejs.org/api/http.html#http_class_http_incomingmessage)
@@ -95,6 +97,8 @@ server.on('request', (req, res) => {
 - `options`: see [Options](#options)
 - `onProxyError(err)`: called on error
 
+See [`upgrade`](https://nodejs.org/api/http.html#http_event_upgrade)
+
 ### Options
 
   - `hostname`: proxy [`http.request(options)`](https://nodejs.org/api/http.html#http_http_request_options_callback) target hostname