Add developer guidance for API changes to CustomRuns

[lbernick]

CustomRun is different from other Tekton APIs because its API is implemented by custom run controllers, rather than in-tree controllers. Therefore, adding new fields to this API should be done with extra caution. This commit adds guidance for Tekton devs on whether a new field should go in the CustomRun
API or left to custom run controllers to support as part of the custom spec or custom status, based on whether a feature is applicable to all CustomRuns or just a feature of some of them.

/kind documentation

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• n/a Has Tests included if any functionality added or changed
• Follows the commit message standard
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• n/a Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[wlynch]

/lgtm

[tekton-robot]

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[lbernick]

/retest