docs: Add an example of chrome extension testing on CI

[mshustov]

Cases:
1 Run tests locally. There is a good example already https://github.com/GoogleChrome/puppeteer/blob/master/docs/api.md#working-with-chrome-extensions
2 Run test on CI. That part is not covered, as extension testing cannot be run in headless mode. I’ve spent a couple of hours to manage to run puppeteer on TravisCI and then found and adopted your setup https://github.com/GoogleChrome/puppeteer/blob/master/.travis.yml
Is that a recommended way to run puppeteer in headful mode? Should we add it to the docs?

[aslushnikov]

Is that a recommended way to run puppeteer in headful mode?

@restrry If you're talking about xvfb - than yes, that's the default solution to run x server applications in headless environment.

Should we add it to the docs?

I'd appreciate a PR for this. We have a travis CI section in our troubleshooting.md