Document admission criteria / process for adding new logs

[mhutchinson]

@FiloSottile has added a spicy package to Debian that makes it super easy for people to create a new log. A log should be witnessed to be more useful than a signature, and thus we might expect an increase in the number of requests to add logs to be witnessed.

This repo has the omniwitness policy that is used by the armored witness and any non-armored omniwitness deployments. This is a natural home for policy on admission criteria.