compress/lzw: add commentary that TIFF's LZW differs from the standard algorithm.
compress/lzw: add commentary that TIFF's LZW differs from the standard
algorithm.

See https://codereview.appspot.com/105750045/ for an implementation of
TIFF's LZW.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/102940043

webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc