Add a "Webhooks" section to the README.md

[cbrgm]

Related: #2347

This PR adds a "Webhooks" section to the README.md and provides an example snippet.

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[codecov]

Codecov Report

Merging #2353 (7f985b0) into master (0fef117) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2353   +/-   ##
=======================================
  Coverage   98.05%   98.05%           
=======================================
  Files         119      119           
  Lines       10471    10471           
=======================================
  Hits        10267    10267           
  Misses        140      140           
  Partials       64       64           

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[gmlewis]

Thank you, @cbrgm !
LGTM.
Merging.