docs(cookbook - custom form controls) Add cookbook for custom form controls with ngModel.

[bennadel]

This cookbook demonstrates how to use value accessors to connect custom
components to the robust Form functionality that Angular provides with its
ngModel, ngControl, and ngFormControl directives.

[mattdistefano]

Possible to incorporate the ARIA checkbox design pattern here so this is a little friendlier to users with disabilities?

[bennadel]

@mattdistefano Sorry, just saw your comment. I am not too familiar with any of the ARIA stuff (unfortunately). I will try to take a look.

@wardbell I removed the WIP from the title.

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We found a Contributor License Agreement for you (the sender of this pull request) and all commit authors, but as best as we can tell these commits were authored by someone else. If that's the case, please add them to this pull request and have them confirm that they're okay with these commits being contributed to Google. If we're mistaken and you did author these commits, just reply here to confirm.

[Foxandxss]

This cookbook needs RC5 update, and I guess Forms update (not sure about this).