docs: README needs a short description of what tsping does

As a newcomer to `tsping` (but a long-time fan of cake-autorate), I don't quite understand what `tsping` does. The README would benefit from a short introductory paragraph that says what it does, and a display of its output for `-t` (and maybe `-e`, if it's substantially different). For example (I am making this up...):

> # Introduction
>
> `tsping` sends ICMP echo/timestamp requests to one or several IP hosts. It displays the results of those requests like this:
>
> ```
> tsping IP1 IP2 IP3
> ... whatever the output looks like ...
> ```

I would also swap the **Usage** and **Compilation** sections since most readers will want to know about the options before they need to compile it. Many thanks.

Start with README and review the existing Usage and Compilation sections, along with the documented -t and -e options. Add a short introduction and representative tsping output, reorder the sections as requested, and confirm the README explains what the command does and how its output looks.

Written by the indexing model from the issue text.