Add examples of how to add channel priorities to environment files.

[davidrpugh]

Need explicit discussion of how to add channel priorities to environment files. Typical way is to add a channels section to your environment.yml file.

name: null

channels:
  - conda-forge
  - defaults

...

Priority is from highest to lowest, so in the above the conda-forge channel would be given highest priority.

[samumantha]

mentioned something about this in PR #52

[davidrpugh]

@samumantha I saw that as well. I have some things to say about this as well but don't have time to include them before the workshop tomorrow so I will leave this issue open to remind myself to add my thoughts later.