Document using @peter-evans' create-or-update-comment with this Action

[solvaholic]

Fixes #41

• Summarize findings from add_pr_comment adds new comment for each run #41
• Demonstrate recommended configs in a public repo
• Document them such that they can be applied in others' workflows
• Either prove a safe way to trigger on pull requests from forks (see Improve comment formatting #67), or document they're not handled

https://github.com/marketplace/actions/create-or-update-comment

[solvaholic]

Summarize findings from #41

• Let the built-in add_pr_comment feature stay simple
• Document the create-or-update-comment approach, including how to update the previous comment rather than create a new one
  (check out add_pr_comment adds new comment for each run #41 (comment))
• Unrelated to this pull request, add plan output as an output
  (as suggested in add result output #11 and add_pr_comment adds new comment for each run #41 (comment))

[solvaholic]

Using @xt0rted's example as a guide and @travislikestocode's edits for #66, I stubbed out this reference config:

https://github.com/solvahol/dns/blob/850ba9cca0753de939dba41e0a05bb1bbb26ba72/.github/workflows/octodns-sync.yml

Which passed with empty payload and a fake --doit:

https://github.com/solvahol/dns/runs/2984436036?check_suite_focus=true

Demonstrate recommended configs in a public repo

Next, I think:

• Compare solvahol/dns's octodns-sync.yml again to @xt0rted's example, see what I missed
• Add pull_request trigger and add PR comment via create-or-update-comment
• Either prove a safe way to trigger on pull requests from forks (see Improve comment formatting #67), or document they're not handled
  Edit: Removed this to-do item to the main issue body.

[solvaholic]

Updated reference workflow made happy results in a PR over in solvahol/dns.