Version of JS supported

[TomasBarry]

This issue has been brought up before (Poltergeist not supporting ES6): #823

However it should be made explicit in the README what version of JS is supported by Poltergeist. We have spent a significant amount of time debugging a silent issue that came down to the version of JS used (meaning we could not use includes or startsWith).

Thanks.

[twalpole]

This is a limitation of the version of PhantomJS being used. If you want to submit a PR making that clear in the README (which no one actually reads) it will be considered