Include a Table of Contents in the readme

[mattmoor]

We should call out the constituent elements of this repo that we expect a "complete" source to implement. This currently includes the types and controller structure, but we should also include:

docs: #118
cli plugin: #117

... and anything else we might expect folks building a source to implement.

cc @lionelvillard @evankanderson @n3wscott @vaikas @nachocano

[vaikas]

I think there's probably something about observability, say things like tracing information and so forth.

[mattmoor]

Conformance testing: #120

[lionelvillard]

pinging @lberk since he's been looking at this in the past.

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[lberk]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[lberk]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.