Update documentation for wrappers

[tjdett]

Wrappers with languageServerHaskell.useCustomHieWrapperPath were removed in 0.0.40, but unfortunately README.md still specifies using them instead of languageServerHaskell.hieExecutablePath.

This is confusing for anybody with a custom wrapper, as the upgrade path from 0.0.39 is not immediately obvious.

Unfortunately this may not be a quick find-and-replace, as the section "Using multi-root workspaces" does not appear to have a straight-forward alternative.

[jneira]

@tjdett thanks for note it, docs has been updated to remove the custom wrappers related options with #227