Document Maven release process

We need to document the release process for Maven releases of Onionmasq, so mutliple people can publish the packages here in our Gitlab maven registry.

Nathan linked to:

• https://gitlab.torproject.org/tpo/core/onionmasq/-/blob/main/android/OnionmasqAndroid/build.gradle?ref_type=heads#L34
• https://gitlab.torproject.org/tpo/core/onionmasq/-/blob/main/android/OnionmasqAndroid/onionmasq/publishLocal.gradle?ref_type=heads
• ./gradlew publishToMavenLocal etc etc