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, I'm not sure that this repo needs it. As this repo grows, we may want to enable this here.

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

[benjagm]

Let's do this.

[heysujal]

@benjagm is this still relevant?

[0priyamdey]

Sir, I would also like to contribute to the project.