a few examples in docs

[choldgraf]

Hello 👋 per some of my experiences in #65 , I thought it would be helpful to add two examples to the documentation. This PR does the following:

1. Adds code to demonstrate a custom encoder with a simple Altair example
2. Adds code to demonstrate using the display encoder with Matplotlib figures
3. Adds a link to the docs in the README because I missed it the first time :-)

Let me know if this is helpful, or if folks would like to see changes!

closes #65

[codecov[bot]]

Codecov Report

Merging #66 into master will not change coverage. The diff coverage is n/a.

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[MSeal]

Thanks for posting the PR -- I'll let you read the comments I left on the issue and see how you'd want to update the doc additions.

[choldgraf]

Looking at your comments, I think that these doc additions could be useful. This PR gives an example that shows off the pattern that you suggested (using 'display' to store only the display information), as well as another example to show how to create a custom encoder that stores information about the plot itself. I think both examples would have helped me come to this conclusion more quickly. What do you think?