docs: troubleshooting notes for DirectoryBackground

[dwmkerr]

Fixes #68.

[codecov]

Codecov Report

Merging #250 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #250   +/-   ##
=======================================
  Coverage   12.54%   12.54%           
=======================================
  Files          86       86           
  Lines        3020     3020           
=======================================
  Hits          379      379           
  Misses       2641     2641

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