Add docs about tile size variants

[rmarianski]

This should only get merged once the buffered terrain service is in production, and we are ready to go public with it.

[rmarianski]

@nvkelso I was pretty terse with what I added. Could you take a quick look and add in some more details as you see fit?

[nvkelso]

This change won't show up on mapzen.com/documentation until we do a release of this repo:

• https://github.com/mapzen/documentation/blob/master/Makefile#L6

So we can merge this as-is now.

[nvkelso]

@rmarianski can you also add documentation in the vector-datasource repo about the optional 256 and 512 sizes?

[rmarianski]

@rmarianski can you also add documentation in the vector-datasource repo about the optional 256 and 512 sizes?

Just double checking, you mean just for the tile sizes, and nothing to do with terrain right?