Update SystemD.rst, adding KillSignal to service unit
Without this systemctl stop emperor.uwsgi.service with uwsgi 1.9/2.0 was
taking an age for me. This change ensures systemd sends SIGQUIT and not
SIGTERM as recommended in:
https://uwsgi-docs.readthedocs.org/en/latest/ThingsToKnow.html
Merge pull request #137 from cjmayo/systemd

Update SystemD.rst, adding KillSignal to service unit