doc: troubleshooting += Failed to read ng output after 5 seconds

(sapling split of 42ea5a4ab21d9c1dfbd2c6e1bfb4756cc276872e)

src/python/twitter/pants/docs/tshoot.rst

@@ -35,4 +35,24 @@ want to track down exactly what, you can reset your state::
     $ build-support/python/clean.sh # clean cached files
     $ ./pants goal ng-killall --ng-killall-everywhere # stop background procs
 
+*****************
+Nailgun 5 seconds
+*****************
+
+If Pants fails with a stack trace that ends with something like ::
+
+    File "twitter/pants/tasks/nailgun_task.py", line 255, in _spawn_nailgun_server
+    File "twitter/pants/tasks/nailgun_task.py", line 226, in _await_nailgun_server
+    twitter.pants.java.nailgun_client.NailgunError: Failed to read ng output after 5 seconds
+
+Pants uses a program called nailgun to run some JVM jobs. Pants runs
+nailgun as a server in the background and then sends requests to it. If
+nailgun runs into problems, it might not respond.
+
+To debug this, look in ``./pants.d/ng/*/*``: these files should be named
+``stdout`` and ``stderr``.
+
+One typical cause behind this symptom: if you removed your machine's Ivy cache,
+Pants may try to use symbolic links to files that have gone away.
+To recover from this, :ref:`washpants`.