Document how to add custom JNI bindings (#50)

This is how we add Linux Arm64 JNI bindings.

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
+