Include HACKING.rst in MANIFEST.in

Without this, I'm seeing:

  Running setup.py egg_info for package from file:///.../hacking-0.5.5.6.ga6bef44.zip
    error in setup command: Error parsing ../setup.cfg: IOError: 2

This is a "file not found" when it tries to open HACKING.rst.

Not sure whether something changed recently to cause this to start
happening.

Change-Id: I499332181c83a7bb127261efd6354ae4d92f5709