Better documentation/description

[uuf6429]

Saying "this is a wrapper of X for Y" (in chrome store / github readme) isn't very useful... what should I expect when I install this plugin?

Looking at the code, I noticed two things:

1. for some reason it's hard-coded to use the petstore examlple (/index.js)
2. it's hard-coded to work with github urls ending with yaml only (/background.js) - which is suspicious on two points: openapi files can be anywhere, especially sites exposing an api and it seems to assume that all yaml files are open api files

These points might be fine if someone bothered actually documenting what this thing does.