docs(quickstart): add a section to installing typings manually

Author: Foxandxss

public/docs/ts/latest/quickstart.jade

@@ -141,6 +141,13 @@ block install-packages
   code-example(language="sh").
     npm install
 
+  .l-sub-section
+    :marked
+      The `typings` folder could not show up after `npm install`. If so, please install them manually.
+
+    code-example(language="sh").
+      npm run typings install
+
   .alert.is-important
     :marked
       Scary <span style="color:red; font-weight: bold">error messages in red</span> may appear **during** install.