Add guidelines for writing controllers

[mcmire]

Explanation

So far we have not have any documentation on how to write controllers the BaseController v2 way. These guidelines aim to fill in the missing pieces. Note that while each guideline has examples, we are still missing a complete example controller file which demonstrates all of the guidelines in one go. That will come in a future PR.

References

Progresses #4504.

Changelog

(None)

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate

[mcmire]

I'm not sure how we intend on using these selectors just for state access. I just made up this API, but not sure if reselect or something else would work as well here.

I might drop this section, though, until we've worked out how selectors would work for this use case.

[Gudahtt]

It should probably be fooController.state one line down from here, rather than fooController. We'll need to select from the state. But aside from that, this looks basically like I had imagined as well!

[mcmire]

I ended up updating this example to use reselect.

[mcmire]

This is ready for review again. I've expanded heavily upon the set of guidelines I posted originally, so this may need a complete re-review.

[MajorLift]

Had some minor proofreading comments but looks great overall! Having a clear set of best practices available to reference should reduce a lot of friction and uncertainty around working with controllers.

[MajorLift]

Some suggestions for consistency with previous changes.