README: Clarify unstable between release on an unstable protocol
README: Clarify unstable between release on an unstable protocol

Unstable changes can be made to unstable protocols, but this was about
unstable changes within the same major version.

Signed-off-by: Jonas Ådahl <67dcd8a17e758192e0cbb577ce1d62cc7da6cf49@gmail.com>

wayland-tablet/wayland-protocols,jpetersen/wayland-protocols