Add "How to uninstall elm" section

[pobch]

There is only "How to install elm" section: https://guide.elm-lang.org/install/elm.html

The missing part is how to uninstall it for each OS (Mac, Linux, Windows).

[datafilter]

Uninstall instructions for Linux is there by following the install link on the page.
https://github.com/elm/compiler/blob/master/installers/linux/README.md#uninstall

You'll find the sibling directories document how to uninstall on Mac & Windows.