Mention installing via ghcup in the readme

Thanks @hasufell!

Author: lukel97

README.md

@@ -93,6 +93,12 @@ This is *very* early stage software.
 - For Cabal based projects, both ghc and [cabal-install](https://www.haskell.org/cabal/) must be installed and on the PATH. It can also be installed with [ghcup](https://www.haskell.org/ghcup/) or [Chocolatey](https://www.haskell.org/platform/windows.html) on Windows.
 - For Stack based projects, [stack](http://haskellstack.org) must be installed and on the PATH.
 
+### ghcup
+
+```
+ghcup install hls
+```
+
 ### Visual Studio Code
 
 If you are using Visual Studio Code, the [Haskell extension](https://marketplace.visualstudio.com/items?itemName=haskell.haskell) will automatically download and install `haskell-language-server` for you.