Fix: Add StringProvider to list in README.md

[localheinz]

This PR

• adds StringProvider to the list in README.md

Follows #326.
Follows #328.

[codecov]

Codecov Report

Merging #331 into main will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #331   +/-   ##
=========================================
  Coverage     68.97%   68.97%           
  Complexity       65       65           
=========================================
  Files             8        8           
  Lines           245      245           
=========================================
  Hits            169      169           
  Misses           76       76           

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