docs(readme): document 'pipx' alternative installation steps

[AdrianDC]

Description

docs(readme): document 'pipx' alternative installation steps

Closes: #1113

Checklist

• Add test cases to all the changes you introduce
• Run ./scripts/format and ./scripts/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

Steps to Test This Pull Request

Additional context

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.57%. Comparing base (120d514) to head (a536d31).
Report is 413 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1215      +/-   ##
==========================================
+ Coverage   97.33%   97.57%   +0.23%     
==========================================
  Files          42       55      +13     
  Lines        2104     2516     +412     
==========================================
+ Hits         2048     2455     +407     
- Misses         56       61       +5     

Flag	Coverage Δ
unittests	97.57% <ø> (+0.23%)	⬆️

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[Lee-W]

Overall, looks great! add 2 minor rewording comments