Document custom image transformations

[aniketmaurya]

What does this PR do?

Document custom image transformations

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[codecov]

Codecov Report

Merging #620 (6c4d2ff) into master (4c69c1b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #620   +/-   ##
=======================================
  Coverage   90.63%   90.63%           
=======================================
  Files         173      173           
  Lines        9179     9179           
=======================================
  Hits         8319     8319           
  Misses        860      860           

Flag	Coverage Δ
gpu	?
pytest	?
unittests	90.63% <ø> (ø)

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[ethanwharris]

Awesome work! Few small comments on the text. I think we should move the example to a testcode block, let me know if you need any help with that 😃

[ethanwharris]

LGTM 😃