Document dev setup

Fixes https://github.com/nextcloud/twofactor_u2f/issues/250.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

README.md

@@ -20,3 +20,21 @@ In order to use this app for authentication, you have to use a browser that supp
 
 ## Login with external apps
 Once you enable U2F with Two Factor U2F, your aplications (for example your Android app or your GNOME app) will need to login using device passwords. To manage it, [know more here](https://docs.nextcloud.com/server/11/user_manual/session_management.html#managing-devices)
+
+## Development Setup
+
+This app uses [composer](https://getcomposer.org/) and [npm](https://www.npmjs.com/) to manage dependencies. Use
+
+```bash
+composer install
+npm install
+npm run build
+```
+
+or if you're using [Krankerl](https://github.com/ChristophWurst/krankerl)
+
+```bash
+krankerl up
+```
+
+to set up a development version of this app.