add visual instructions for customizing environment

[scottyhq]

I just took a bunch of screenshots documenting the process for adding conda packages, since it's pretty great you can do this entirely on github without a linux terminal! cc @ArianeDucellier @mdenolle

1. create a new branch

2. select environment.yml file

3. select "edit" button

4. add the package(s) you want

5. commit changes

6. go to main page (https://github.com/UW-ESS-DS/590-Spr21-image/) and create a pull request:

7. can test the new environment via the launch binder button, or merge pull request if tests pass (environment successfully builds). then it will be posted to dockerhub and available to jupyterhub.