Document why chrome is a prerequisite & about possible alternatives

[sivakumar-kailasam]

Now with Firefox working in headless mode we should document this as a part of the guides.

Original inspiration: https://twitter.com/Romgere_dev/status/1050702153302593536?s=19

[kellyselden]

https://twitter.com/kellyselden/status/1051035243011080193

A good Firefox example if you want to use it.

[sivakumar-kailasam]

Thanks @kellyselden

[jenweber]

This ticket is still relevant.

The current blueprint for testem specifies Chrome. here's the example output

To close this issue:

• [ ] Add a new section to https://cli.emberjs.com/release/advanced-use/build-targets/ called "Testing with different browsers"
• [ ] Show the current output of the blueprint
• [ ] Give an example for Firefox and explain what (if anything) needs to be installed.
• [ ] In your PR, link to a minimal working example repo