[FEATURE] Improve README

[WilliamBergamin]

Description of the problem being solved

It will be helpful for users to get information on hooks and how to override them.

Alternative solutions

In the README.md create a section that describes the purpose/function of each hook, and then advises how to override them.

Requirements

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[zimeg]

👋 Heyo! Using this README.md as inspiration for the one over in the Node @slack/hooks (there's so much great info here) and was wondering if the outgoing_domains should be removed from the example manifest?

[WilliamBergamin]

@zimeg Thanks you 🙏
Correct me if I'm wrong (maybe the CLI handles this gracefully) but the manifest.validate endpoint expects an array for this field, last time I checked this field was not nullable