Add ScalarNonlinearFunction to README

[odow]

Closes #638

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.67%. Comparing base (988d7e3) to head (6ea02ce).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #639      +/-   ##
==========================================
- Coverage   94.30%   93.67%   -0.63%     
==========================================
  Files           6        6              
  Lines        2739     2579     -160     
==========================================
- Hits         2583     2416     -167     
- Misses        156      163       +7     

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