Update README to use modern `./install.hs` script only

[fendor]

There are some inconsistencies and artefacts old installation processes.
See haskell/haskell-ide-engine#1626 where this bit a user.
We need to update these instructions!

[flip111]

Maybe you can send a pull request ?

[jneira]

@flip111 @fendor done in #207