fix: only append README.md notice if it doesn't yet exist

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: Migration script re-adds end of README.md notice #843
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Uses the <!-- ... --> line as a detector to know whether to append.

Note that creation was fixed previously, just not appending (updating).

[codecov]

Codecov Report

Merging #845 (cd6abf0) into main (dd2392a) will increase coverage by 0.02%.
Report is 3 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #845      +/-   ##
==========================================
+ Coverage   92.16%   92.19%   +0.02%     
==========================================
  Files          84       84              
  Lines        4226     4240      +14     
  Branches      263      264       +1     
==========================================
+ Hits         3895     3909      +14     
  Misses        331      331              

Flag	Coverage Δ
create	71.82% <50.00%> (-0.15%)	⬇️
initialize	34.82% <100.00%> (+0.21%)	⬆️
migrate	71.21% <50.00%> (+<0.01%)	⬆️
unit	52.71% <100.00%> (+0.22%)	⬆️

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Files Changed	Coverage Δ
src/steps/updateReadme.ts	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[github-actions]

🎉 This is included in version v1.29.46 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀