Expand the README to be a useful base for external extension pages

I just received an additional (after approval) reviewer reply on [the AMO version of Verify Me](https://addons.mozilla.org/en-US/firefox/addon/verify-me/) requesting a more clear description of what the extension does. I added the following:

> Looks for [rel="me" links](https://indieweb.org/rel-me) on the current page, then check if they link back with a rel="me" too. Shows a green ✔️ or red ❌ depending on whether they do or not. Allowing [distributed verification](http://www.kevinmarks.com/distributed-verify.html) of ownership and profile equivalence between different websites and social media profiles.

It would be great if we can work on the README in this repository to contain all the information necessary for external portals like these. That way a simple copy-paste is all that will be needed for future releases.

* [ ] A single line summary of the extension.

AMO summaries are max 250 characters, but shorter seems better. Does the Chrome Web Store have something like it, what is its limit? GitHub has the repo description up top. Not sure if that comes with any length limitations, but GitHub-likes such as Gogs have imposed limits around 250 characters too.

* [ ] A multi paragraph description of the extension.

What does it do, and why would you use it? More so than with the single line summary, a user should be able to decide whether this is worth installing based on the description.

AMO recommends 250 characters, but doesn’t impose any special limit (that is obvious to me). It also supports putting links in here. I think this would be the “Overview” section on the Chrome Web Store? Does that one support external links?

* [ ] Screenshots of the extension in use.

Both AMO and the Chrome Web Store want screenshots. I think these should be put in the repository and also included in our own README. AMO has the following recommendations for screenshots:

> The maximum and recommended size for screenshots is 1280x800 pixels. 640x400 pixels is a good alternative if you only have smaller images.

Start by reading the repository README and compare it with the AMO listing linked in the issue. Add a concise summary, a multi-paragraph explanation with relevant links, and screenshots meeting the stated size guidance. Done means the README contains the information external extension portals need for a future copy-paste release.

Written by the indexing model from the issue text.