Update HIE install instructions

[Gipphe]

With haskell/haskell-ide-engine#1452 merged, HIE doesn't even have a build-all build target any longer, and (seemingly) this build target was discouraged for new users. As such, it might be time to update the install section fo the README with the generally better build target build, meaning you end up installing HIE with

./install.hs build

# Or on Windows
stack ./install.hs build