Mention/list all available browser launch configs

[gekkedev]

The file karma.conf.js is configured for the browser configs Firefox and FirefoxHeadless whilst the Readme features an example karma config using Firefox, FirefoxDeveloper, FirefoxAurora and FirefoxNightly.
Only diving into the code made me find the full list.

module.exports = {
  'launcher:Firefox': ['type', FirefoxBrowser],
  'launcher:FirefoxHeadless': ['type', FirefoxHeadlessBrowser],
  'launcher:FirefoxDeveloper': ['type', FirefoxDeveloperBrowser],
  'launcher:FirefoxDeveloperHeadless': ['type', FirefoxDeveloperHeadlessBrowser],
  'launcher:FirefoxAurora': ['type', FirefoxAuroraBrowser],
  'launcher:FirefoxAuroraHeadless': ['type', FirefoxAuroraHeadlessBrowser],
  'launcher:FirefoxNightly': ['type', FirefoxNightlyBrowser],
  'launcher:FirefoxNightlyHeadless': ['type', FirefoxNightlyHeadlessBrowser]
}

IMHO it would be very useful to list these various configurations in the readme. It's been done for the Chrome-Launcher as well. Might eventually create a PR if approveable.