Adds a doc note about pagerduty API version.

[drob]

Thanks for OSSing awesome tool! It has already saved me a bunch of time. I am a fan.

This item tripped me up on install. Figured I'd document for the next person to come along.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[drob]

Given #14 I think this is moot now.