document how to include under Karma

Due to #521, we can't use our regular build chain with this, so we're having trouble with integrating code that uses this into our test suite.

Can you suggest a way for us to set up our `karma.conf` so that `Box.Preview` is available at test time?

Preferably without having to re-fetch preview.js from the network with every test run.

Or is there a mock implementation of Box.Preview we should load instead?

Start by reviewing the Karma configuration in karma.conf and the integration context described in issue #521. Determine how Box.Preview from preview.js can be made available without fetching it on every test run, or whether a mock is documented. Done means the repository explains a repeatable local setup for the Karma test suite.

Written by the indexing model from the issue text.