Improve README with installation instructions

[petros]

This is missing currently. We assume folks that want to use this, will figure out how to install a release.

It's basically getting the executable and copying it to a folder in your PATH so that it's accessible from your shell.

But that's not obvious, so we need to be explicit about it.

Later down the road, which is not in the scope of this issue, we should consider making it available to places like https://crates.io, https://brew.sh etc.