docs: Add readme hint for library audience

[Leon-hk]

What this PR does / why we need it:

• Add a small hint to inform about the target audience
• See docs: Add ADR 30 about the scope of python-gardenlinux-lib gardenlinux#4210

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.41%. Comparing base (d417011) to head (a743187).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #297   +/-   ##
=======================================
  Coverage   91.41%   91.41%           
=======================================
  Files          49       49           
  Lines        2585     2585           
=======================================
  Hits         2363     2363           
  Misses        222      222           

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[yeoldegrove]

I reworked the README to be shorter and reference the documentation.