Update the documentation

[SimonSimCity]

The documentation of this package is too complex and misses out on things, because more and more issues are opened which could have been solved by a better documentation.

Since this package is quite widely used, the meteor-guide should also be updated: https://github.com/meteor/guide/blob/master/content/testing.md

[SimonSimCity]

It also doesn't contain anything about how to use puppeteer ...

[mitar]

I think how to install peer NPM packages for testing packages should be somehow documented, see: meteor/meteor#8577 (Or maybe Meteor issue should be reopened in the first place.)