Just to add to the conversation,
mr_mcthing - the module has a reed switch/magnetic switch built-in to the module already! This is how you can use a magnet to change the field and then the module can report an open or close.
Nick has done a great job with his setup -Thanks Nick!

We also put together a video using the magnetic switch on our production room door and then relaying that information to an Android phone app using MQTT. It's not as fancy as Nick's - doesnt include a door knocking option.

You can check it out here: