Add GitHub code owners for the specification

[ml-evs]

I raised in the last meeting that it might make sense to add GitHub "code owners" to the optimade.rst file, which are required to review any PRs before merging (instead of the scattergun approach we take at the moment, where the PR author has to choose from a list of 50 people).

This artificially elevates some members of the community, but currently we are vulnerable to features getting pushed through without consensus.

This would simply require creation of a CODEOWNERS file under .github/ following https://docs.github.com/en/free-pro-team@latest/github/creating-cloning-and-archiving-repositories/about-code-owners

This could even be split down by branch (slightly awkwardly requires different CODEOWNERS files on each branch) so that "fixes" to whatever branch ends up tracking v1.0.x (see #340) only requires 1 or 2 reviewers, whereas new features could require multiple code owners to agree (say 1 per major implementation).

There would obviously be an acceptance that new features are therefore dealt with much more slowly.

[merkys]

An alternative proposal would be to increase the number of required reviews (currently 2). However, this would not prevent "gangs" from pushing their proposals 😉, only make that somewhat harder.

[ml-evs]

Coming back to this idea, I think this would be a nice way of handling the develop->master and final release process.

It would probably make sense to have the "nominated" release managers for each release as required reviewers for the final PR to master. We can think about doing this for the next release candidate/final 1.2.0 release.