Update README.md and added CODEOWNERS

[BigLep]

Making clear the repo status per ipfs/boxo#218

Note that no "deprecated types" are being added. There is only a general notice that this repo is on a deprecation path and a pointer to the code copy that Kubo uses. Any code that @ipfs/kubo-maintainers own will switch to the Boxo version. A CODEOWNERS was added to clarify who will look at any PRs or do mandatory fixes here (likely in sync with @ipfs/kubo-maintainers since both sides are generally only doing critical changes here.)

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[BigLep]

2023-03-28 conversation with @rvagg : he is going to:

1. Better explain the landscape
2. Adjust the CODEOWNERS list

[rvagg]

Updated this PR, making myself the only "CODEOWNER" (let me know if you really want to be in that party, but I'm assuming that nobody else really wants to) and changed the framing in the README to:

This library is maintained, but not actively developed. It will continue to receive fixes and security updates for users that depend on it. However, it may be deprecated in the future and it is recommended that you use alternatives to the functionality in go-merkledag, including:

...with a list of links including the kubo fork

Will merge if I don't get feedback in a couple of days.

[BigLep]

Great updates @rvagg! Let's merge. Followups can always be made if needed.

[BigLep]

I'm going to merge since @rvagg and I have reviewed and I want to close out ipfs/boxo#218. If there are additions/changes they can get made after.