Meson is pretty well tested and works in most configurations now, so we
can remove the warning about it being unsuited for actual use.

It's also worth documenting that meson 0.42.0 or greater is required.

v2: - Minor rewording of supported platforms as suggested by Emil
    - Add two missing tags as reported by xmllint --html

Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com> (v1)
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> (v1)