compress/zlib: mention that NewReaderDict can return ErrDictionary
compress/zlib: mention that NewReaderDict can return ErrDictionary

Fixes issue 7935.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, ruiu
https://codereview.appspot.com/147390043

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