Add 'changining the port' section to the README

Author: robatron

README.md

@@ -55,3 +55,11 @@ algernon server.lua
 ```
 
 And visit <http://localhost:3000/>. Try opening multiple tabs!
+
+## Changing the port
+
+You can change the port number by setting the `$PORT` environment variable before invoking any of the scripts above, e.g.,
+
+```sh
+PORT=3001 node server.js
+```