Add instructions for using module

Add instructions for using module, either on the module page on D.o, or in a README file.

Suggested wording:

1. After installing the module, navigate to the "Manage display" page where the image field is.
If you're using Media, this might be Structure > Media types > Image > Manage display.

2. Change the image field's formatter from "Image" to "Image (Exclude GIF Processing)"