Document a good way to make GIFs for the module pages

[Frijol]

As mentioned in https://github.com/tessel/t2-start/issues/16, we need to update our output GIFs. We let them slide with the out-of-date terminal for a while, but now it's pretty blatant because the hardware looks different:

Like this one from http://tessel.github.io/t2-start/modules/rfid.html:

We should document a method in the README for how to do this.

I made these original ones. Here's what I remember:

• Video from a camera
• Screen captures made in Quicktime
• I made the terminal text super CTRL + + so it would show up in a tiny GIF
• Spliced together videos in probably iMovie or something
• These excellent command line tools http://parkerhiggins.net/2012/10/howto-create-an-animated-gif-from-a-video-with-command-line-tools/
• Manually deleting frames to make them smaller - yes I know there's a better way
• Host the images on S3, though now we use Imgur (see https://github.com/tessel/t2-start/issues/16)

To complete this issue, PR the readme with a process as good as or better than mine. :dancer: