Add GIFs to README

[award999]

Other extensions use GIFs in the README to show interaction on the extension's marketplace page and in the extensions view of VSCode

Examples:
https://marketplace.visualstudio.com/items?itemName=redhat.java
https://marketplace.visualstudio.com/items?itemName=ms-python.python

[rbenegal]

I think we can reuse the GIFs from the walkthrough to keep the extension size smaller, and I can add a new one to show code completion. Will put up a patch after the walkthrough goes in.