[COG] Very basic port of spec to github done

[Chris Holmes]

Apologies for the delay on this, and it's really the bare minimum, but I
just ported the text from
https://trac.osgeo.org/gdal/wiki/CloudOptimizedGeoTIFF to
https://github.com/cogeotiff/cog-spec/blob/master/spec.md

I wanted to do a lot more to clean it up, but decided to not let good be
the enemy of great, and to ask for help. I figure we can start with the
exact text that's on the wiki, and then evolve the spec through pull
requests.

I gave Even and Norman rights on it, and we can expand that out as people
make pull requests. I'll try to put in some more time this week to get it
to a decent state.

And then we can start proposing recommendations / best practices /
'profiles'.

C
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