NEWS: Document "nmbug: Translate to Python"

NEWS: Document "nmbug: Translate to Python"

For more details, see the commit message for 7f2cb3be (nmbug:
Translate to Python, 2014-10-03).  I realized while writing this that
the 7f2cb3be commit message has:

  * 'nmbug log' now execs 'git log', as there's no need to keep the
    Python process around once we've launched Git there.

But we dropped that exec in favor of the subprocess approach between
v3 and v4, I just forgot to update the commit message [1].

[1]: id:e630b6763e9d0771718afee41ea15b29bb4a1de8.1409935538.git.wking@tremily.us
     http://article.gmane.org/gmane.mail.notmuch.general/19007