docs: note persist-credentials requirement for actions/checkout@v6

[postalservice14]

Summary of changes

actions/checkout@v6 changed how it stores the credentials it persists: they now
live under $RUNNER_TEMP instead of the local git config. A Docker container action
like this one cannot read them by default, so git push-based tagging fails with:

fatal: could not read Username for 'https://github.com/': No such device or address

This adds a README note under the existing fetch-depth note explaining the fix:
set persist-credentials: true (with an Actions runner ≥ v2.329.0), or rely on
the default GIT_API_TAGGING: true so tags are pushed via the GitHub API instead of
git push.

This is the recurring cause behind #350, #352 and #358 — none of which are bugs in
the action itself, only a documentation gap.

Breaking Changes

Do any of the included changes break current behaviour or configuration?

NO — documentation only, no change to entrypoint.sh or action.yml.

How changes have been tested

• Confirmed the diagnosis against the credential-handling change documented in the
  actions/checkout v6 release notes and the reproductions in actions/checkout v6 breaks GH API #352 / Different outcome depending on GIT_API_TAGGING is true of false #358.
• Verified the new Markdown renders and the issue links resolve.
• No code paths changed, so existing tests are unaffected.

List any unknowns

• None. If maintainers prefer, the note could also be inlined as a comment in the
  example workflows in the README rather than (or in addition to) the standalone note.