README: Add a link to a fork with Unicode support

[unclechu]

It would be useful for the people who want this but didn’t realize there already is an implementation they can use and not spend time implementing it themselves.

For the context you can follow these:

• https://github.com/neovimhaskell/haskell-vim/pull/106
• https://github.com/neovimhaskell/haskell-vim/pull/134
• https://github.com/neovimhaskell/haskell-vim/issues/41
• https://github.com/neovimhaskell/haskell-vim/issues/108

Also ping @Achierius, @DeCentN2Madness, and @mkohlhaas. You might find this fork useful.

[unclechu]

@raichoo by the way, it seems test010 is broken. See https://github.com/wenzel-hoffman/haskell-with-unicode.vim/runs/8289172006?check_suite_focus=true Don’t mind the other failed tests, those are I added myself which are similar to test010.