Enable branch protection

[gregsdennis]

For the other repos in the org, we are enabling branch protection to:

• prevent pushing directly to the primary branch (requires a PR)
• require an approval on the PR.

However, since this repo is quite new and only has a single contributor, I'm not going to do that yet. As this repo grows, we will want to enable this.

See json-schema-org/json-schema-spec#1258 for more info.