Add examples of --select to the README.

[jonathanj]

Fixes #37

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This change is 

[codecov-io]

Codecov Report

Merging #91 into master will decrease coverage by <.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   55.76%   55.75%   -0.01%     
==========================================
  Files          23       23              
  Lines        1406     1408       +2     
  Branches      188      189       +1     
==========================================
+ Hits          784      785       +1     
- Misses        614      615       +1     
  Partials        8        8

Flag	Coverage Δ
#unittests	55.75% <33.33%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/eliottree/__init__.py	100% <ø> (ø)	⬆️
src/eliottree/_render.py	88.09% <0%> (ø)	⬆️
src/eliottree/filter.py	95.65% <50%> (-4.35%)	⬇️

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