support nix run and add basic doc

flake.nix

@@ -148,5 +148,12 @@
           cli = rocBuild.roc-cli;
           lang-server = rocBuild.roc-lang-server;
         };
+
+        apps = {
+          default = {
+            type = "app";
+            program = "${rocBuild.roc-cli}/bin/roc";
+          };
+        };
       });
 }

getting_started/nix.md

@@ -1,7 +1,11 @@
 # Roc installation guide for Nix
 
-
-## NixOS quick start
+If you want to quickly get started you can do
+```shell
+nix run roc-lang/roc -- <roc args>
+# example nix run roc-lang/roc -- repl
+```
+to try out roc!
 
 <details>
 <summary>

nix/builder.nix

@@ -3,6 +3,8 @@ let
   inherit (compile-deps) zigPkg llvmPkgs llvmVersion llvmMajorMinorStr glibcPath libGccSPath;
 
   subPackagePath = if subPackage != null then "crates/${subPackage}" else null;
+
+  mainBin = if subPackage == "lang_srv" then "roc_ls" else "roc";
 in
 rustPlatform.buildRustPackage {
   pname = "roc" + lib.optionalString (subPackage != null) "_${subPackage}";
@@ -84,4 +86,11 @@ rustPlatform.buildRustPackage {
         ${wrapRoc}
       fi
     '';
+
+  # https://ryantm.github.io/nixpkgs/stdenv/meta/
+  meta = {
+    homepage = "https://www.roc-lang.org/";
+    license = lib.licenses.upl;
+    mainProgram = mainBin;
+  };
 }