Document script for updating hpcc user's keys

Script in execution question is:

sudo /opt/HPCCSystems/sbin/update-keys -g

Reasons for documenting:

1. We recommend that the user/installer run the script during a new installation, so we should provide information about what it does.
2. Periodically rotating keys may be a good idea, in a general sense. This may not come up in real life, though.
3. If an admnistrator has several clusters that should talk to each other, it is important that they understand why the keys are important and further to know when clusters should use the same keys and when they should not.