Update CODEOWNERS

[baentsch]

The CODEOWNERS file triggering review(er)s seems to have fallen out of maintenance.

This issue is to suggest

• bringing it back in line with current contributors/contributions
• documenting the need to keep the file current when doing major merges

[SWilson4]

For what it's worth, open-quantum-safe/tsc#77 will make the CODEOWNERS file valid again by providing @crockeea and @alexrow with "write" access. This doesn't remove the need to update the file to reflect current code ownership, but it does improve the state of affairs.