Document that custom helpers can be used for anything, not just components

[benshine]

Currently, the only description of writing custom helpers is in the section on components:
http://ember-cli-page-object.js.org/docs/v1.4.x/components#custom-helper

I found out in emberjs slack from @jeradg that "The custom helpers will work in components, page objects, collections, etc."

It would be great if that info were in the actual documentation :)

[san650]

Thanks for the feedback, it's really useful to help us improving the addon.

[jeradg]

@benshine @san650 Time for me to take that Slack wisdom and put it in the repo!

[jeradg]

Note (also from a conversation with @benshine on Slack): We also need documentation that you can add plain JS methods to the page object. In those methods, you can refer to the PO with the this keyword, return this to allow for method chaining, etc.