message: [java] Add note about requiring Java 8.
diff: This is a **work-in-progress** Java wrapper for [Indy](https://www.hyperledger.org/projects/indy). It is implemented using a foreign function interface (FFI) to a native library written in Rust. Indy is the
  open-source codebase behind the Sovrin network for self-sovereign digital identity.
  +This Java wrapper currently requires Java 8 (e.g. the openjdk-8-jdk package in Debian/Ubuntu).
  +
  Pull requests welcome!
  **Not ready for production use! Not all commands work properly! Use at your own risk!**
  