Add Codespace instruction to README.md

[eeholmes]

Thanks for the great package! It was the one thing that worked to reduce my PDFs!

Codespaces is now free and one can run your package with a few clicks. I added a section with instructions. Since you can upload and download files to a Codespace, you install with a click or two, upload and then download.

Here is the instructions, in case they are useful.