Basic ropes on git stash command

Quick overview on what `git stash` is and some of the basic
commands to run with it.

Going with `git stash pop` as the default means of
extracting from the stack instead of `git stash apply`.
Former feels like the path of least surprise by taking
out what you've just put in.

As requested in #36

_layouts/reference.html

@@ -51,6 +51,7 @@
               <li><a href="/basic/#commit">commit</a></li>
               <li><a href="/basic/#reset">reset</a></li>
               <li><a href="/basic/#rm-mv">rm, mv</a></li>
+              <li><a href="/basic/#stash">stash</a></li>
             </ul>
           </div>