Add httpcache code example to README.md

[madneal]

Relates to: #2631.

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[gmlewis]

Thank you, @madneal !
Just a couple minor tweaks, please.

[codecov]

Codecov Report

Merging #2632 (e8019a0) into master (34cc7b4) will not change coverage.
The diff coverage is n/a.

❗ Current head e8019a0 differs from pull request most recent head f5c4d07. Consider uploading reports for the commit f5c4d07 to get more accurate results

@@           Coverage Diff           @@
##           master    #2632   +/-   ##
=======================================
  Coverage   98.03%   98.03%           
=======================================
  Files         130      130           
  Lines       11144    11144           
=======================================
  Hits        10925    10925           
  Misses        150      150           
  Partials       69       69           

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[gmlewis]

Thank you, @madneal !
LGTM.
Merging.