Add note about extensions registering themselves

[johnthagen]

Addresses #579 (comment)

[codecov]

Codecov Report

Merging #581 (45e675e) into master (59f9749) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #581   +/-   ##
=======================================
  Coverage   98.80%   98.80%           
=======================================
  Files          58       58           
  Lines        6702     6702           
=======================================
  Hits         6622     6622           
  Misses         80       80           

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[tfranzel]

thanks! i'll adapt the warning because i put some work into making that import loop go away.