PR Labeler: label PRs which update **/README.md as type/docs

[jswank]

What this PR does / why we need it:
Ensure that PRs which modify **/README.md are labeled appropriately with the type/docs label. See #11753 for details.

Which issue(s) this PR fixes:
Fixes #11753

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[JStickler]

@jswank I wasn't sure after our exchange of comments on your issue, did you want to move ahead on this PR? I don't approve PRs outside of /docs/sources but I can ask one of the engineers to review/approve this.

[jswank]

@JStickler My understanding is that you and docs team are the primary beneficiaries of the PR. If your workflow is improved by labelling PRs updating README.md for review by your team, then moving ahead makes sense. If that is not beneficial to you, then the issue and this PR can be closed.

[JStickler]

@trevorwhitney @jeschkies Tagging you because you were the last ones to update these files.
@periklis Tagging you for visibility as this touches the operator files.

[trevorwhitney]

don't know a ton about this action, but based on a quick reading of https://github.com/actions/labeler this lgtm!