Document image management

[j08lue]

          This is almost done, the tests have been merged. Needs a paragraph or so of docs to be called 'done'

Originally posted by @yuvipanda in #16 (comment)

[batpad]

Creating a check-list here of things we need to document:

• Add deprecation notice to https://github.com/NASA-IMPACT/veda-jh-environments, link to new docs
• Add Image Management section to docs in veda-jupyterhub repo
• Document working with https://github.com/nasa-IMPACT/pangeo-notebook-veda-image/
• Document creating custom images for workshops, etc. - @maxrjones cookie cutter repository as an option
• Document other images used on hub: R and QGIS images

cc @sunu