add description of '3dobject' support.

Hi,

A number of websites in the online 3d community have come together to define a common extension of Open Graph for 3D objects.  I've written it up here:

http://exocortex.com/blog/opengraph_3dmodel_extension

I've also made this PR that adds this to the formal Open Graph website as one of the community agreed to definitions.

Best regards,
Ben Houston