add nextflow instruction (#177)

jupyter-nextflow/nextflow-welcome.html

@@ -48,6 +48,7 @@ <h2 class=header>Get started with Nextflow</h2>
     needs. To get started:
   </p>
   <ul>
+    <li>If you want your files to persist, first navigate to the "pd" folder.
     <li>Open a new "Launcher" tab and open a terminal.
     <li>You can find your sample configuration file in the "data" directory.
       Copy it to a new "nextflow.config" file by running this command in the