expand README

README.md

@@ -1,4 +1,35 @@
-# qrbill
+# qrbill Go package
+
+<img src="https://github.com/stapelberg/qrbill/raw/master/2020-06-25-qr-mary-jane.png" width="200" height="200" align="right">
+
+The qrbill package implements generating QR-bill QR codes, which is a Swiss
+🇨🇭 payment standard:
+
+* [launch
+announcement](https://www.six-group.com/en/newsroom/media-releases/2020/20200609-qr-bill-launch.html)
+
+* [Specification: Swiss Implementation Guidelines
+  QR-bill](https://www.paymentstandards.ch/dam/downloads/ig-qr-bill-en.pdf)
+\
+  (version 2.1 from 30 September 2019 at the time of writing)
+
+These QR codes fill in details of a bank wire transfer, either all fields or
+just some of them.
+
+Recipients can scan the QR code with their mobile banking app, or with their
+online banking web site’s mobile scan functionality, making it much easier to
+send a wire transfer.
+
+An example use case is for an association or club: you can send out payment
+instructions to your members to collect membership fees (“Einzahlungsschein”).
+
+## Validity
+
+The [SIX QR-bill validation
+portal](https://validation.iso-payments.ch/gp/qrrechnung/home) successfully
+confirms that e.g. the code at the top right is valid.
+
+## qrbill API (Example program)
 
 To install the program:
 
@@ -6,8 +37,28 @@ To install the program:
 go install github.com/stapelberg/qrbill/cmd/...
 ```
 
-To run the program:
+To run the program, run `qrbill-api` in the terminal. You will see an output like this:
 
 ```
-qrbill-api
+% qrbill-api
+2020/06/25 23:32:52 QR Bill generation URL: http://localhost:9933/qr?format=html
 ```
+
+You can append the URL parameters on the left to customize the code. E.g.:
+
+http://localhost:9933/qr?format=html&udname=Mary+Jane&udaddr1=Artikel+19b
+
+When you open the URL in your web browser, you should see the following debug
+output:
+
+![](2020-06-25-format-html.jpg)
+
+When you scan the resulting QR code with your eBanking app, you should see a
+form like this:
+
+<img src="https://github.com/stapelberg/qrbill/raw/master/2020-06-25-ebanking-app.jpg" height="300">
+
+Once you are happy with the code parameters, change the `format` parameter from
+`html` to `png`, e.g.:
+
+http://localhost:9933/qr?format=png&udname=Mary+Jane&udaddr1=Artikel+19b