Initial documentation setup

[NoaMills-USDA]

Adds initial versions of files/directories needed for readthedocs documentation.

Once merged, I request that the repo host takes the following steps to create a readthedocs project:

• Create a readthedocs account, and link it to your github account. Note: when I created a readthedocs account, the verification email was blocked by IT. I recommend emailing spam.abuse@usda.gov and requesting that they don't block emails from support@readthedocs.org
• From your new readthedocs account, click "Projects" from the top right dropdown menu, then "Import a project". Select the pygcdl repo if it pops up, or manually type in the url to the pygcdl repo. Name the project "pygcdl".