Add notice to default mentioning to build yarn.nix

Also correct output generation

Author: expipiplus1

default.nix

@@ -1,15 +1,17 @@
 { pkgs ? import <nixpkgs> {} }:
 with pkgs;
-pkgs.mkYarnPackage {
-  name = "vscode-haskell";
+
+# Please run `yarn import` first to generate yarn.lock!
+pkgs.mkYarnPackage rec {
+  name = "haskell";
   src = ./.;
   packageJSON = ./package.json;
   yarnLock = ./yarn.lock;
 
   installPhase = ''
     mkdir -p "$out/dist"
     yarn vscode:prepublish --output-path "$out/dist"
-    mv deps/vscode-haskell/{package.json,hie-vscode.sh,hie-vscode.bat} "$out"
+    mv deps/${name}/package.json "$out"
   '';
 
   distPhase = ''