Add instructions for installing PWA

[scbarton]

I'm really excited about this project because it potentially eliminates a ton of pain points around markdown, particularly editing tables, and can potentially take advantage of many more HTML features than markdown.

Is your feature request related to a problem? Please describe.
First, though, I needed to get the app installed. Some directions about installing PWAs on Edge or Chrome would be helpful. Not a web dev so I had never done that before.

Describe the solution you'd like
A paragraph in the README.md would probably do.

Describe alternatives you've considered
I did a bunch of googling to figure out that I could not use a PWA with Safari, and used the "Apps" item in the "..." menu of Edge.

Additional context
Again, thanks for getting this started, I hope it gains traction and becomes your new, high-paying job.