update runbook to explain how to bring polyfill.io back if the cache dissappears and the servers are knocked offline

[JakeChampion]

Currently this is not documented anywhere

We should document the steps within the runbook and then go through it with the Ops team so that they know how to bring polyfill.io back online if this scenario occurs

The steps are:

1. Record the original dyno size of the origami-polyfill-service-eu origami-polyfill-service-us applications
2. scale up the applications to the largest dyno size and the maximum amount of dynos to be able to handle the traffic whilst the cache is repopulated
3. once the cache-hit ratio looks good (98% or higher) on the Fastly dashboard, then scale down the applications to the original size