Made Note About gem propshaft in README.md

[Ted-Tash]

What is this pull request for?

Adding a note about propshaft compatibility for older rails versions, with encouragement to use alchemy ~> 8.0.0a if on `Rails >8. Taken from this comment.

Closes #3330

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change --> this is a documentation only change 😃

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.99%. Comparing base (6afea50) to head (297ca35).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3386   +/-   ##
=======================================
  Coverage   96.99%   96.99%           
=======================================
  Files         277      277           
  Lines        7262     7262           
=======================================
  Hits         7044     7044           
  Misses        218      218           

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[tvdeyen]

Thanks

[Ted-Tash]

@tvdeyen with the note block:

[tvdeyen]

Thanks