Improve readme and submodules

[tmcw]

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This fixes two issues I encountered on my first run of this firmware:

• Git doesn't clone submodules by default, so this advises people to. This was reported earlier in https://github.com/tidbyt/hdk/issues/19#issuecomment-2353661299
• Notes that only HTTP urls, not HTTPS, are supported as remotes.

[tidbyt-bot]

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[tmcw]

I have read the CLA Document and I hereby sign the CLA