update release script and BUILD.md

BUILD.md

@@ -8,10 +8,25 @@ Make sure that `JAVA_HOME` and `ANDROID_HOME` are set correctly.
 ```
 git clone https://github.com/jrpie/Launcher
 cd Launcher
-./gradlew build
+
+./gradlew assembleDefaultRelease
 ```
 
-Note that you need to sign the apk.
+This will create an apk file at `app/build/outputs/apk/default/release/app-default-release-unsigned.apk`.
+
+Note that you need to sign it:
+```
+apksigner sign --ks "$YOUR_KEYSTORE" \
+    --ks-key-alias "$YOUR_ALIAS" \
+    --ks-pass="pass:$YOUR_PASSWORD" \
+    --key-pass="pass:$YOUR_PASSWORD" \
+    --alignment-preserved \
+    --v1-signing-enabled=true \
+    --v2-signing-enabled=true \
+    --v3-signing-enabled=true \
+    --v4-signing-enabled=true \
+    app-default-release-unsigned.apk
+```
 
 
 See [this guide](https://developer.android.com/build/building-cmdline)