clarify that showvalues "name" can be a string

[stevengj]

In the README.md example, it used a symbol for the "name" in the showvalue argument. It seems a lot clearer to use a string in the example — not only are strings less obscure than symbols, they are also more flexible and natural as labels.

Internally, the code calls string(name), so it can be anything.

(Right now, showvalues is only documented in the README … would be nice to have this in a docstring somewhere.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.73%. Comparing base (807496a) to head (721da1a).
Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   93.48%   96.73%   +3.25%     
==========================================
  Files           1        1              
  Lines         399      552     +153     
==========================================
+ Hits          373      534     +161     
+ Misses         26       18       -8     

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