Document usePolling option in karma.config.js

Expected behavior

Seeing all available options that materially affect the test harness here

Actual behavior

I don't see usePolling; which solves the problem of auto-watching mounted source files in Docker containers; cf. this SO thread.

Enviroment Details

• Karma version (output of karma --version): 0.13.9
• Relevant part of your karma.config.js file

    ...
    // won't work in a Docker container
    autoWatch: true,

    autoWatchBatchDelay: 1000,

    usePolling: true,
    ...

This makes it work with the following output from docker-compose logs, e.g.