add pre-commit.ci badge

[bjlittle]

This PR adds the pre-commit.ci badge to the README.md as a follow on to #1934

Controversially, I've also removed the autofix_prs disable in the .pre-commit-config.yaml - totally happy to encourage debate on this choice.

Personally, the pre-commit.ci autofix does save the exact situation that we're initially in, where we banked a PR that wasn't compliant for all the git-hooks (#2146 - my bad, I should have rebased to include #1934 and the git hooks would have detected non-compliance locally).

Naively, I trust the hooks to do the "right thing" and auto-fix as part of the pre-commit.ci workflow. I'd love to hear the flip argument as to why we shouldn't adopt that pattern.

[bjlittle]

@greglucas Can we please, please, please disable stickler-ci 🙏

I'd be soooo happy to do that 😄

I'd argue that our goal now is to fully adopt the pre-commit.ci workflow, given #1934 has landed i.e., put all our eggs in the pre-commit git-hook basket, and therefore we can purge the unnecessary use of the stickler-ci service. It's served its purpose - time to retire it, I say 😉

Thoughts?

[bjlittle]

pre-commit.ci autofix