Scripts readme (rebased onto develop)



This is the same as gh-47 but rebased onto develop.

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To simplify the omero-user-scripts README, https://github.com/ome/omero-user-scripts/pull/3 has removed all specific directories. Instead, those have been moved here as a general introduction, though perhaps that info lives best on plone.

/cc @hflynn, @jburel, @will-moore