add a read me for bumping libxmtp (#201)

Author: nplasterer

library/src/main/java/README.md

@@ -0,0 +1,15 @@
+# LibXMTP Kotlin
+
+Kotlin code emitted by the `bindings_ffi` crate in [libxmtp](https://github.com/xmtp/libxmtp) including how to get jni libraries
+
+## Process for updating from a [libxmtp](https://github.com/xmtp/libxmtp) Kotlin Binding Release (work in progress!)
+
+1. From repo [libxmtp](https://github.com/xmtp/libxmtp) checkout the branch you would like to make a release from
+2. Navigate to the `bindings_ffi` folder
+3. Run `./gen_kotlin.sh`
+4. Run `./cross_build.sh`
+5. Copy the contents of `libxmtp/bindings_ffi/src/uniffi/xmtpv3/xmtpv3.kt` to `xmtp-android/library/src/main/java/xmtpv3.kt`
+6. All instances of `value.forEach` should be changed to `value.iterator().forEach` to be compatible with API 23
+7. Copy the jniLibs from `libxmtp/bindings_ffi/jniLibs` to `xmtp-android/library/src/main/jniLibs`
+
+You should now be on the latest libxmtp. Tests will fail if the jniLibs do not match the version of xmtpv3.