Add warmup.cfg, configuring collective.warmup for production deployments.

The warmup.cfg automatically configures collective.warmup to hit the Plone
site root and its resources on instance starts.

It can be used in combination with production.cfg, where no further configuration
is needed.
Merge pull request #15 from 4teamwork/jone-warmup

Add warmup.cfg, configuring collective.warmup for production deployments