Minimal doc about running tests for the gem

[olance]

Not much, but the info is missing and could avoid errors when trying to run tests for the first time ^^

[zpao]

nit: PhantomJS, and probably worth making that a link.

[jtmalinowski]

How about we link to https://github.com/teampoltergeist/poltergeist with a hint to head there?

[jtmalinowski]

@olance my bad it took me this much time to respond, just let me know if you are still going to finish this PR

[olance]

@JakubMal no worries, I do not have much free time on hand either ^^
So yes, I'd like to finish this PR, just let me know: would you go with an example test too? Do I need to rebase the PR?

[zpao]

Just address the comments but ignore the example test part. You can do it in a new commit or amend your current commit and rebase, your call.

[olance]

👍

[rmosolgo]

Sorry for leaving so long, for a while there's been a short instruction on how to run the tests in the Readme:

Feel free to rebase & open another PR if you think that documentation should be extended!