url for old npm warning

bin/warnings.sh

@@ -29,6 +29,6 @@ warn_old_npm() {
   local npm_version=$1
   if [ "${npm_version:0:1}" -lt "2" ]; then
     local latest_npm=$(curl --silent --get https://semver.herokuapp.com/npm/stable)
-    warning "This version of npm ($npm_version) has several known issues - consider upgrading to the latest release ($latest_npm)"
+    warning "This version of npm ($npm_version) has several known issues - consider upgrading to the latest release ($latest_npm)" "https://devcenter.heroku.com/articles/nodejs-support#specifying-an-npm-version"
   fi
 }