Establish clear ownership of js-multiformats

[BigLep]

Background

The lack of unified consensus of #249 highlighted that there is a lack of clarity on the ownership of this repo. CODEOWNERS isn't defined, and there isn't an understanding of how owners should handle disagreements. Some of the main contributors of this repo over the last 12 months are individuals who do this on the side of other team work they have (@rvagg and @Gozala - thank you!). Others help maintain here as part of their full-time maintenance and development of libraries that consume this repo (e.g., js-libp2p, Helia).

Done criteria

1. Create CODEOWNERs
2. Document what is expected of a CODEOWNER
3. Document where triage and synchronous decisions for this repo are handled

Proposal

1. CODEOWNERs for now should be existing maintainers who have stayed involved with the project (i.e., @rvagg, @Gozala) or consuming projects that have stayed involved in the project (js-libp2p, Helia). (It's fine if any of these individuals/groups want to back out.). Lets handle this in set up CODEOWNERS for js-multiformats github-mgmt#83
2. In the absence of another venue currently and the general change happening to PL EngRes teams as they "nucleate" out from PL, I'd like to propose that we house triage and synchronous conversations as part of js-libp2p triage since it is a multiple organization venue that meets weekly. (We know this time is not conducive for everyone, so we are also happy to find an alternative venue.)
3. Concerning the expectations of codeowners, I don't think we need to be too prescriptive here, but at the minimum, I think they should engage on open issues and PRs when pinged within a week. Ideally they would also proactively engage on issues and PRs and participate in triage.

### Proposed actions
- [ ] Create CODEOWNERs: https://github.com/multiformats/github-mgmt/pull/83
- [ ] Update README with triage info: https://github.com/multiformats/js-multiformats/pull/274

[BigLep]

Another couple of comments:

1. It's recognized that the number consumers of this repo is large (https://github.com/multiformats/js-multiformats/network/dependents?dependent_type=PACKAGE&package_id=UGFja2FnZS0xMTU3NjI2MTU5 and https://www.npmjs.com/browse/depended/multiformats ). As a result, changes need to be made with care here.
2. I'm not proposing more formal governance at the moment given changes here tend to be "low".
3. The proposal doesn't really talk about how someone else becomes a codeowner. I think that would ultimately be up to the majority vote of the current codeowners (where codeowner teams count as a single vote).

[rvagg]

ACK and 👍