README: Remove 'Getting involved' contributing guide link
README: Remove 'Getting involved' contributing guide link

The exercism/docs "contributing-to-language-tracks" is mostly a rehash
of the contributing guide in the Haskell track's README.md. It does
elaborate in some areas, and then it is less specific in others. We
still link to it under 'Update an exercise test suite'.

exercism/xhaskell