docs: Make `main_binary` front and center
docs: Make `main_binary` front and center

The README was changed to progressively show how to launch a binary,
starting with `main_binary`.  Sadly, `main_binary` needs to be skipped
because skeptic has the cargo stuff locked.

`execute` was removed from the README since it seemed like a lesser use
case.

Fixes #61

killercup/assert_cli