Add scikit-image to requirements.txt (atlasmaker)

[sebastianbk]

The package scikit-image turns out to be required in order to make use of the atlasmaker tool. So I added it to the requirements file.

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[sebastianbk]

I signed it!

[googlebot]

CLAs look good, thanks!

[jameswex]

thanks for this. can you also add it to the README.md for atlasmaker, which also describes the requirements individually?