Document how to add custom JNI bindings (#50)

This is how we add Linux Arm64 JNI bindings.

publishing/PUBLISHING.md

@@ -7,6 +7,7 @@ To publish snapshot, you must add your sonatype credentials for the `ossrh` serv
 
 - Download `snapshot.zip` generated by the `Publish snapshot` github action
 - unzip `snapshot.zip` in the `publishing` directory
+- add additional JNI bindings (optional, see below)
 - edit `secp256k1-kmp-snapshot-deploy.sh` and update the `VERSION` environment variable if needed
 - run `secp256k1-kmp-snapshot-deploy.sh`
 
@@ -17,7 +18,25 @@ You must edit `secp256k1-kmp-staging-upload.sh` and add your sonatype credential
 
 - Download `release.zip` generated by the `Publish release` github action (which is triggered every time you publish a github release)
 - unzip `release.zip` in the `publishing` directory
+- add additional JNI bindings (optional, see below)
 - edit `secp256k1-kmp-staging-upload.sh` and update the `VERSION` environment variable if needed
 - sign all artifacts with a valid gpg key: `find release -type f -print -exec gpg -ab {} \;`
 - run `secp256k1-kmp-staging-upload.sh`
 - log into sonatype, close and publish your staging repository. Artifacts will be available on Maven Central within a few hours.
+
+## Adding custom JNI bindings
+
+Github CI currently generates JNI bindings for Windows x64, Linux x64 and iOS x64. But it is possible to add custom bindings to JNI packages before 
+they are published to maven central. This is how we add linux arm64 bindings:
+- compile JNI bindings for Linux Arm64 (on a Linux Arm64 machine, cross-compilation is not supported)
+  - git clone --recursive https://github.com/ACINQ/secp256k1-kmp.git
+  - cd secp256k1-kmp 
+  - TARGET=linux ./native/build.sh 
+  - mkdir -p jni/jvm/build/linux 
+  - TARGET=linux ./jni/jvm/build.sh 
+  - JNI library is: jni/jvm/build/linux/libsecp256k1-jni.so
+- copy libsecp256k1-jni.so to fr/acinq/secp256k1/jni/native/linux-aarch64/libsecp256k1-jni.so
+- run `secp256k1-kmp-add-linuxarm64.sh` and specify either `release` or `snapshot` and the `VERSION` environment variable, for example:
+  - VERSION=0.6.4-SNAPSHOT ./secp256k1-kmp-add-linuxarm64.sh snapshot
+  - VERSION=0.6.3 ./secp256k1-kmp-add-linuxarm64.sh release
+

publishing/secp256k1-kmp-add-linuxarm64.sh

@@ -0,0 +1,17 @@
+#!/bin/bash -x
+
+if [ $# -eq 0 ]
+  then
+    echo "specify either snapshot or release"
+    exit 1
+fi
+
+# add aarch64 (ARM64) library to the linux jar
+if [ -e fr/acinq/secp256k1/jni/native/linux-aarch64/libsecp256k1-jni.so ]
+then
+    jar -uf $1/fr/acinq/secp256k1/secp256k1-kmp-jni-jvm-linux/$VERSION/secp256k1-kmp-jni-jvm-linux-$VERSION.jar fr || exit
+else
+    libsecp256k1-jni.so for arch64 is missing
+    exit 1
+fi
+