Android building instructions

cozo-lib-java/README.md

@@ -4,13 +4,40 @@ This crate provides the JNI bindings for using Cozo in Java/JVM languages/Androi
 
 You do not use this crate directly. Instead, use:
 
-* ... for Android
 * ... for Java or other JVM languages
 * ... for Clojure on JVM (you can also use the Java library, but this one is nicer)
+* ... for Android
+
+Keep reading only if the prebuilt binaries provided by these libraries do not suit your needs.
 
-## Building
+## Building for JDK
 
 With the Rust toolchain installed,
 ```bash
-cargo build --release  -p cozo_java -F compact -F storage-rocksdb
+cargo build --release -p cozo_java -F storage-rocksdb
+```
+
+## Building for Android
+
+Building for Android is not easy, and we will be very sketchy.
+
+The first thing to note is that you should omit `-F storage-rocksdb` from the build command above,
+unless you are prepared to manually change lots of `build.rs` flags in 
+[cozorocks](../cozorocks) to build the RocksDB dependency.
+
+Then, in addition to adding Android targets to the Rust toolchain, 
+you also need to set up the Android NDK
+cross-compilation and libraries paths, etc.
+This is notoriously hard to get right, but fortunately 
+you can just use the Docker image [here](https://github.com/cross-rs/cross)
+which has everything set up for you.
+
+When everything is set up correctly, the following command show complete without errors:
+
+```bash
+for TARGET in aarch64-linux-android armv7-linux-androideabi i686-linux-android x86_64-linux-android; do
+  cross build -p cozo_java --release --target=$TARGET
+done
 ```
+
+For running on modern Android phones, the single target `aarch64-linux-android` is probably enough.