Add CODEOWNERS

[braised-babbage]

This adds a CODEOWNERS file, which we can use to notify appropriate people of PRs.

From the docs linked above,

Code owners are automatically requested for review when someone opens a pull request that modifies code that they own. Code owners are not automatically requested to review draft pull requests. For more information about draft pull requests, see "About pull requests." When you mark a draft pull request as ready for review, code owners are automatically notified. If you convert a pull request to a draft, people who are already subscribed to notifications are not automatically unsubscribed. For more information, see "Changing the stage of a pull request."

At the moment, I am just adding "global" owners. The two listed owners reflect representatives from two organizations using OpenPulse. The working group may wish to add others.

Every code owner must have write access to the repository. So we will need to provide that access to @jcjaskula-aws

[hodgestar]

I can't comment on the set of owners but seems nice to have. Also happy to have myself added although I'm watching the whole repository in any case.