Document usePolling option in karma.config.js

### Expected behavior

Seeing all available options that materially affect the test harness [here](http://karma-runner.github.io/0.13/config/configuration-file.html)
### Actual behavior

I don't see `usePolling`; which solves the problem of [auto-watching mounted source files in Docker containers](https://github.com/docker/docker/issues/18246); cf. this [SO thread](http://stackoverflow.com/questions/28681491/within-docker-vm-gulp-watch-seems-to-not-work-well-on-volumes-hosted-from-the-h).
### Enviroment Details
- Karma version (output of `karma --version`): 0.13.9
- Relevant part of your `karma.config.js` file

``` javascript
...
// 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.



Start with the documented configuration options linked in the issue and inspect the repository's karma.config.js references to understand how usePolling is presented. Add usePolling to the configuration documentation, including its Docker file-watching use case, and verify that the option appears alongside the other test-harness settings.

Written by the indexing model from the issue text.