[Doc] Fix typo in format doc, make variable-length format more clear. Close #55
[Doc] Fix typo in format doc, make variable-length format more clear. Close #55

chris-b1/feather,chris-b1/feather,wesm/feather,wesm/feather,wesm/feather,chris-b1/feather,wesm/feather,chris-b1/feather,chris-b1/feather,wesm/feather,wesm/feather,chris-b1/feather,wesm/feather,wesm/feather