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

[joshtrichards]

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

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