Add README.md with (very) basic instructions

README.md

@@ -0,0 +1,25 @@
+# Inköpslista
+
+## Building
+
+### Desktop
+```
+cd gui/
+cargo run --release
+```
+
+### Web
+Use the Dockerfile.
+
+```
+docker build . -t <tag>
+```
+
+### Android
+First, install an Android SDK and NDK, for example using Android Studio.
+SDK 35 (Android 15) is known to work. NDK 29 is known to work.
+
+```
+cd gui/
+cargo apk build --target aarch64-linux-android --lib --release
+```