Add a helpful message for when someone does "node npm.js"

npm.js

@@ -1,6 +1,14 @@
 
 process.title = "npm"
 
+if (require.main === module) {
+  console.error(["It looks like you're doing 'node npm.js'."
+                ,"Don't do that.  Instead, run 'make install'"
+                ,"and then use the 'npm' command line utility."
+                ].join("\n"))
+  process.exit(1)
+}
+
 var npm = exports
   , config = require("./lib/config")
   , set = require("./lib/utils/set")