README: clarify the need for both meta-overc and meta-cube
README: clarify the need for both meta-overc and meta-cube

Signed-off-by: Bruce Ashfield <284cc737d174a260eec2a2a6324a304f84b1ae0f@windriver.com>

masselstine/meta-overc,jwessel/meta-overc,jwessel/meta-overc,OverC/meta-overc,WindRiver-OpenSourceLabs/meta-overc,OverC/meta-overc,mufeng/meta-overc,masselstine/meta-overc,mufeng/meta-overc,OverC/meta-overc,jwessel/meta-overc,OverC/meta-overc,WindRiver-OpenSourceLabs/meta-overc,masselstine/meta-overc,OverC/meta-overc,mufeng/meta-overc,OverC/meta-overc,masselstine/meta-overc,WindRiver-OpenSourceLabs/meta-overc