Follow RFC 9309: allow /robots.txt implicitly and reference the RFC in the README

[AdrianAtZyte]

Resolves #35

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.66%. Comparing base (ddf86e9) to head (5fd83d9).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   97.64%   97.66%   +0.01%     
==========================================
  Files           5        5              
  Lines         382      385       +3     
  Branches       78       79       +1     
==========================================
+ Hits          373      376       +3     
  Misses          5        5              
  Partials        4        4              

Files with missing lines	Coverage Δ
src/protego/_protego.py	96.91% <100.00%> (+0.05%)	⬆️

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[codspeed]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing AdrianAtZyte:rfc9309 (5fd83d9) with master (ddf86e9)}