Improve description and information on README

[kcpevey]

Description

I kept seeing mention of npe2 in issues so I googled it and ended up at this repo. I did see on the readme that there is documentation for this here:https://napari.org/stable/plugins/index.html
but when I read the README itself... I still had no idea what npe2 was or even what the letters in the acronym stand for.

I suggest a brief explanation of what npe2 is and what it does be added to the README. And I suggest that a link to appropriate napari docs be added to the "About" section of the github page.