update readme

README.md

@@ -1,3 +1,13 @@
 # nfc-onboarder
 
-This application prepares an NFC tag to be used with our [inventory tracker](https://github.com/creatorsgarten/inventory).
+This application prepares an NFC tag by writing a URL to it. You can configure which URL to write to the tag by setting the `?url=` query parameter in the URL of the page.
+
+## Usage
+
+Construct a URL like this:
+
+```
+https://creatorsgarten.github.io/nfc-onboarder/?url=https://grtn.org/inventory/nfc_{sn}
+```
+
+Replace `https://grtn.org/inventory/nfc_{sn}` with the URL you want to write to the NFC tag. The `{sn}` part will be replaced with the serial number of the NFC tag. Note that the serial number will not have `:` characters in it.