Update README.markdown

Author: tfausak

README.markdown

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 # docker-haskell
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+This Docker image provides a Haskell development environment with the following tools:
+
+- [GHCup](https://www.haskell.org/ghcup/)
+- [GHC](https://www.haskell.org/ghc/)
+- [Cabal](https://www.haskell.org/cabal/)
+- [Stack](https://docs.haskellstack.org/en/stable/README/)
+- [HLS](https://haskell-language-server.readthedocs.io/en/latest/)