Extend nix explanations in README (#549)

* Extend nix explanations in README

* Correct ghcide-nix url

Co-authored-by: Domen Kožar <[email protected]>

Co-authored-by: Domen Kožar <[email protected]>

Author: maralorn

README.md

@@ -31,7 +31,17 @@ There are more details about our approach [in this blog post](https://4ta.uk/p/s
 
 #### With Nix
 
-[See ghcide-nix repository](https://github.com/hercules-ci/ghcide-nix)
+Note that you need to compile `ghcide` with the same `ghc` as the project you are working on.
+
+1. If the `ghc` you are using matches the version (or better is) from `nixpkgs` it‘s easiest to use the `ghcide` from `nixpkgs`. You can do so via
+   ```
+   nix-env -iA haskellPackages.ghcide
+   ```
+   or e.g. including `pkgs.haskellPackages.ghcide` in your projects `shell.nix`.
+   Depending on your `nixpkgs` channel that might not be the newest `ghcide`, though.
+
+2. If your `ghc` does not match nixpkgs you should try the [ghcide-nix repository](https://github.com/cachix/ghcide-nix)
+   which provides a `ghcide` via the `haskell.nix` infrastructure.
 
 #### With Cabal or Stack