add hint about compiling qr code lib to readme

README.md

@@ -33,7 +33,9 @@ and only exists to deliver some static files.
 ### Developing
 
 You can clone this repository, edit the files and open `index.html` -
-as everything's static, you can test and play around.
+as everything's static[^1], you can test and play around.
 Even "Open Chat" should work if Delta Chat is installed.
 
 However, note, that local or other sever's links cannot open Delta Chat directly.
+
+[^1] if you want to change the qr code library `qr.min.js`, you instead need to edit it's source in src and then run `npm run build-qr-generator` to recompile it.