added prebuilt shared objects for libsignal-client

ext/libraries/libsignal-client/README.md

@@ -0,0 +1,18 @@
+# HOWTO BUILD
+
+[cross](https://github.com/rust-embedded/cross) is used for cross compiling [libsignal-client](https://github.com/signalapp/libsignal-client).
+
+* download new release from `https://github.com/signalapp/libsignal-client/releases`
+* unzip + change into directory
+* cd into `java` directory
+* run `cross build --target x86_64-unknown-linux-gnu --release -p libsignal-jni`
+  
+  run `cross build --target armv7-unknown-linux-gnueabihf --release -p libsignal-jni`
+  
+  run `cross build --target aarch64-unknown-linux-gnu --release -p libsignal-jni`
+to build the library for `x86-64`, `armv7` and `arm64`
+* the built library will be in the `target/<architecture>/release` folder 
+
+## Why?
+
+Building libsignal-client every time a new docker image gets released takes really long (especially for cross platform builds with docker/buildx and QEMU). Furthermore, due to this bug here (https://github.com/docker/buildx/issues/395) we would need to use an ugly workaround for that right now. As libsignal-client isn't released very often I guess it's okay to manually build a new version once in a while.