docs(README): Expand details for devs/contributors + misc tweaks

[joshtrichards]

Closes #503

Builds on latest README changes from #539. Attempts to add notes useful to those looking to make changes to the Updater or understand its internals and how it fits in with the other pieces.

Also:

• Corrects a couple typos
• Adds some additional relevant config parameters
• Some minor structural tweaks

Lastly:

• Further refinements can be built on this foundation as needed.
• Appropriate bits will eventually make there way into the Nextcloud Admin Manual

[come-nc]

I’m not a fan of the "Possible future work" section, feature requests should go in issues, not in the README.
Also, things as vague as "Improve documentation" or "Performance optimizations" are useless, they are possible future work of any project out there.

[joshtrichards]

Though just the README, test-master will fail until #543 + nextcloud/password_policy#566 are merged.

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