Adds `json` reporter type to README.

It seems that older versions of Karma (0.10.x at least) would output
`coverage-final.json` by default. Newer versions no longer do this and
require the reporter type to be specified.

See for instance the issue in
[`grunt-istanbul-coverage`](https://github.com/daniellmb/grunt-istanbul-coverage/issues/14)
that was caused by this.