android readme and flags

examples/android/README.md

@@ -1,6 +1,46 @@
 # Bevy OpenXR Android example
 
-Running on Meta Quest can be done with https://github.com/rust-mobile/cargo-apk and requires disabling default features. 
+## Setup
+Get libopenxr_loader.so from the Oculus OpenXR Mobile SDK and add it to `examples/android/runtime_libs/arm64-v8a`
+https://developer.oculus.com/downloads/package/oculus-openxr-mobile-sdk/
+`examples/android/runtime_libs/arm64-v8a/libopenxr_loader.so`
+
+Running on Meta Quest can be done with https://github.com/rust-mobile/cargo-apk. 
 ```sh 
 cargo apk run --release
-```
+```
+
+## Notes
+
+### Relase mode
+More optimisations enabled in Cargo.toml for the release mode. 
+This gives more performance but longer build time.
+```toml
+[profile.release]
+lto = "fat"
+codegen-units = 1
+panic = "abort"
+```
+
+### Cargo apk
+If you see error like `Error: String `` is not a PID`, try to install cargo apk with a fix in branch.
+```sh
+cargo install --git https://github.com/rust-mobile/cargo-apk --branch=adb-logcat-uid
+```
+
+### Temporary JNIEnv log
+This message is logged every frame. It's not yet fixed.
+```sh
+I JniUtils-inl: Creating temporary JNIEnv. This is a heavy operation and should be infrequent. To optimize, use JNI AttachCurrentThread on calling threa
+```
+
+### Android keystore
+Release mode requires keystore. See Cargo.toml `package.metadata.android.signing.release`.
+
+When creating your own apps, make sure to generate your own keystore, rather than using our example one!
+You can use `keytool` like so:
+```sh
+keytool -genkey -v -keystore my-release-key.keystore -keyalg RSA -keysize 2048 -validity 10000
+```
+For more information on key signing and why it's so important, check out this article:
+https://developer.android.com/studio/publish/app-signing