README: clarify libweston purpose/goals.
README: clarify libweston purpose/goals.

v2: Rewrap, add a couple of missing words (Pekka).
v3: Use alternative wording (Yong).

Signed-off-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Reviewed-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

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