Backport PR #5630: Fixed Issue #4012 Added Help menubar link to Github markdown doc

Closes https://github.com/ipython/ipython/issues/4012

IPython/html/templates/notebook.html

@@ -229,6 +229,7 b' class="notebook_app"'
                             ("http://nbviewer.ipython.org/github/ipython/ipython/tree/2.x/examples/Index.ipynb", "Notebook Help", True),
                         ),(
                             ("http://docs.python.org","Python",True),
+                            ("http://help.github.com/articles/github-flavored-markdown","Markdown",True),
                             ("http://docs.scipy.org/doc/numpy/reference/","NumPy",True),
                             ("http://docs.scipy.org/doc/scipy/reference/","SciPy",True),
                             ("http://matplotlib.org/contents.html","Matplotlib",True),