[Docs] Create documentation on how to choose a tag for pangeo-docker images

[jmunroe]

Given the LEAP hub exclusively uses pangeo images, work on documentation on how to choose an image and tag. This should exist upstream, and be broadly useful.

Tasks

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1. Add a readthedocs setup pangeo-data/pangeo-docker-images#473
   +0
2. Add a "How to" on 'finding my current image tag'
3. Add a "how to" on 'finding list of packages in any given tag', using packages.txt on GitHub
4. Add a "how to" on 'finding diff of packages between any two tags', using the github

Not in scope

Documentation on how to choose an image is specifically not in scope for this task, so it can be completed within this sprint.

[jmunroe]

I've started to draft a markdown document that answers the three questions:

• How to find my current image tag
• How to finding list of packages in any given tag
• How to finding diff of packages between any two tags

I'll include a command line solution and a Python snippet solution. Nice see that a RTD setup has be added to Pangeo Docker Images -- I'll submit this markdown doc as a PR to that repo when finished.

[yuvipanda]

@jmunroe great, the PR upstream has been merged and now available at https://pangeo-docker-images.readthedocs.io. Do ping me when you submit a PR there!