* doc: ensure all top dirs have an explanatory README
This makes the repository a lil bit nicer to newcomers.
Part of https://github.com/ooni/probe/issues/1335
* fix: re-run bindata to embed the README
The readme is small, so we can pay the price of adding it.
On a related note, I am very pleased the Go team implemented the
`//go:embed` feature, so we can get rid of this bindata thing.
