Document rough edges. Closes #114

[davewsmith]

Notes that device:read and an entry in tagOwners: in the policy file is required for this action to work.

This, I nope, draws the rest of the owl for some people who've found the stale blog post but have failed to get the action working.

Closes #114.

[oxtoacart]

The required scope (auth_keys) is now documented in the README. The README already links to a KB article on how to use and manage tags, which includes additional nuances that are worth knowing. We prefer not to repeat that information here, as it may also change over time.