Add HTML text informing user about limitations of reading from external imaging files

[codecov[bot]]

Codecov Report

Merging #289 (6b6bef3) into master (5b78887) will decrease coverage by 0.09%. The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
- Coverage   66.27%   66.19%   -0.09%     
==========================================
  Files          35       35              
  Lines        3443     3452       +9     
==========================================
+ Hits         2282     2285       +3     
- Misses       1161     1167       +6     

Flag	Coverage Δ
unittests	66.19% <33.33%> (-0.09%)	:arrow_down:

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Impacted Files	Coverage Δ
nwbwidgets/image.py	48.45% <33.33%> (-1.55%)	:arrow_down:

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