Readme around raise_signal_exceptions option

Allow increased visibility around raise_signal_exceptions option which isn't obviously documented. This is helpful particularly for those using Heroku which uses SIGTERMs quite liberally.

Various people have noticed this, and if it were documented it'd negate the need for opening further issues:

http://stackoverflow.com/questions/7080547/delayed-job-locking-but-not-processing

http://stackoverflow.com/questions/10438100/long-running-delayed-job-jobs-stay-locked-after-a-restart-on-heroku