README: Document versioning scheme, forward compatibility
README: Document versioning scheme, forward compatibility

Signed-off-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Reviewed-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>
Reviewed-by: Quentin Glidic <241385a0e41ea713327e189bce1062decf7c1eb5@sardemff7.net>

sir-murray/weston,krezovic/weston,krezovic/weston,sir-murray/weston