Add all contributors table and instructions for how to update

[edwardchalstrey1]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
• Your branch is up-to-date with the target branch (it probably was when you started, but it may have changed since then).
• You have marked this pull request as a draft and added '[WIP]' to the title if needed (if you're not yet ready to merge).

⤴️ Summary

Adds README table and badge, plus instructions that link to https://allcontributors.org/docs/en/cli/overview

[JimMadge]

Good idea (why haven't we done this already!?!).

If you install the CLI you should be able to test this locally.

[edwardchalstrey1]

Closing this for now as per Martin's comment on slack:

I approved and then disapproved the all-contributors bot as it needs write access to code, which we’ve decided we won’t allow for the DSH codebase as part of our controls over who can make changes to the codebase.

@JimMadge I guess if you were keen to have this then this can reopened later after a discussion with the team

[JimMadge]

@edwardchalstrey1 Good point, the distinction between pinned actions and an unpinned bot is a little subtle but important.

However, I think we can still use all contributors and should reopen this PR. We can commit to using the all contributors CLI to manager the contributors table. I think that is still better than what we currently do. Any workflows we could add to ensure consistency or try and spot missing contributors would be great.

The contributing docs could be updated to say don't use the bot, but ask a maintainer.

[JimMadge]

This is looking great.

A couple of things I think we should do,

• On the preview the contributors page appears under a "More" drop down in the menu bar. Can we make it more visible? Maybe there is a theme option for number of items before the drop down?
• I think we should take the approach of low threshold to contribution. There are more contribution types that we could acknowledge. Would you like me to have a go at updating the contributions?
• It would be worth asking in our Slack channels for contributors we might have missed. Especially contributions which don't appear in the commit history like funding, training, conversations, bug reports.

[edwardchalstrey1]

On the preview the contributors page appears under a "More" drop down in the menu bar. Can we make it more visible? Maybe there is a theme option for number of items before the drop down?

Done