message: README: Update to better describe Tock kernel version support
diff: Rust userland library for Tock (WIP)
  -Tested with tock [Release 1.4.1](https://github.com/tock/tock/commit/7e37bf67761d83fd585cace4fb201e2864d300b1).
  +Generally this library was tested with tock [Release 1.5](https://github.com/tock/tock/releases/tag/release-1.5).
  +Since then changes have been made that might not work with the Tock
  +release 1.5, but instead target Tock master. For example this library
  +might support newer boards (Apollo3), changed boards (HiFive1 revB) or
  +new drivers (HMAC).
  The library works in principle on most boards, but there is currently the [showstopper
  bug #28](https://github.com/tock/libtock-rs/issues/28) that prevents
  