Confusing error when building with older Golang

Trying to build with an older Golang results in a weird error

```
build github.com/dfinity/keysmith: cannot load io/fs: malformed module path "io/fs": missing dot in first path element
```

Having a note of this in the readme would have saved some Googling (since apparently Golang 1.13, at least, ignores the `go 1.16` in `go.mod`)