chore: add build locally section

README.md

@@ -10,6 +10,18 @@ You may get a play-protect warning, and that's because I haven't yet signed the
 
 I will publish it soon on F-Droid and Play store.
 
+## Build locally
+
+If you don't have android and want to build for your own system, you can use the `flutter build` command.
+
+```shell
+flutter build <platform> --dart-define "API_URL=URL_TO_BACKEND"
+```
+
+If you haven't hosted the [backend](https://github.com/arnu515/simpleauthenticator/tree/master/backend) yourself, you can use <https://d13c320db282.up.railway.app>.
+
+To find what platforms are supported, run `flutter build --help`
+
 ## Credits
 
 - Logo Icon: <a target="_blank" href="https://icons8.com/icon/40348/lock">Lock</a> icon by <a target="_blank" href="https://icons8.com">Icons8</a>