Add note about start directory for 'tour' notebook.

docs/examples/notebooks/00_notebook_tour.ipynb

@@ -20,6 +20,9 b''
       "The rest of the notebooks in this directory illustrate various other aspects and ", 
       "capabilities of the IPython notebook; some of them may require additional libraries to be executed.", 
       "", 
+      "**NOTE:** This notebook *must* be run from its own directory, so you must ``cd``", 
+      "to this directory and then start the notebook, but do *not* use the ``--notebook-dir``", 
+      "option to run it from another location.", 
       "", 
       "The first thing you need to know is that you are still controlling the same old IPython you're used to,", 
       "so things like shell aliases and magic commands still work:"