Update readme after "update Renovate version" regex preset is created

[HonkingGoose]

Split off from #722.

we probably should suggest pin and provide a regex manager to auto update as preset

I like this idea, here's a really rough draft for it. 😄

Todo:

• Decide on pinning to just a v1.2.3 version, or to a full checksum
• Create a Regex Manager Preset to help users update their GitHub Action Renovate version
• Link to the new Regex Manager Preset docs
• Create Renovate config example in a json codeblock which uses the new preset

@viceice wanted this to go in a seperate PR, so I'm making this issue to track the work.

[rarkins]

I would prefer we build in awareness for some common "with" dependencies such as renovate and node - not as regex managers

[HonkingGoose]

I found some related issues:

• pin version in github-actions renovate#7516
• Renovate doesn't update NodeJS version used by GitHub actions renovate#7716
• Update GitHub actions runner automatically renovate#19827
• Put our Renovate's org regex managers into public regex manager presets renovate#22025