README: Add a feature and the requirement section

Signed-off-by: kfei <kfei@kfei.net>
Add a lock file checking when rTorrent starts up

Signed-off-by: kfei <kfei@kfei.net>
Deal with the signals

1) Now the `Ctrl-q` in rTorrent can quit normally.
2) `docker stop` also works.

Signed-off-by: kfei <kfei@kfei.net>