add description of '3dobject' support.

[bhouston]

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

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[bhouston]

I've now signed the agreement. :)

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