README.md: Explain that "unstable" label is not semantical
As evidenced by thread https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/6161#note_1781172 protocols marked as "unstable" are still treated by some implementors as unstable. Close this loophole.
Signed-off-by: Mikhail Gusarov dottedmag@dottedmag.net