Add NumPy nbviewer link to README

README.md

@@ -32,7 +32,7 @@ The top-level directory contains materials for the following lessons:
 2. Introduction to Python ([slides 1][lesson_02_01], [slides 2][lesson_02_02],
    [slides 3][lesson_02_03])
 3. More Python Goodness ([notebook 1][lesson_03_01], [notebook 2][lesson_03_02])
-4. Working with NumPy arrays
+4. Working with NumPy arrays ([slides][lesson_04])
 5. IPython Notebook ([notebook][lesson_05])
 6. Plotting with matplotlib
 7. Python for data analysis
@@ -45,6 +45,7 @@ The top-level directory contains materials for the following lessons:
 [lesson_02_03]: http://nbviewer.ipython.org/urls/git.lumc.nl/humgen/programming-course/raw/master/02%20-%20Introduction%20to%20Python%20(3).ipynb
 [lesson_03_01]: http://nbviewer.ipython.org/urls/git.lumc.nl/humgen/programming-course/raw/master/03%20-%20More%20Python%20goodness%20(1).ipynb
 [lesson_03_02]: http://nbviewer.ipython.org/urls/git.lumc.nl/humgen/programming-course/raw/master/03%20-%20More%20Python%20goodness%20(2).ipynb
+[lesson_04]: http://nbviewer.ipython.org/urls/git.lumc.nl/humgen/programming-course/raw/master/04%20-%20Working%20with%20NumPy%20arrays.ipynb
 [lesson_05]: http://nbviewer.ipython.org/urls/git.lumc.nl/humgen/programming-course/raw/master/05%20-%20IPython%20Notebook.ipynb
 [lesson_09_01]: http://nbviewer.ipython.org/urls/git.lumc.nl/humgen/programming-course/raw/master/09%20-%20A%20sip%20of%20Biopython%20(1).ipynb
 [lesson_09_02]: http://nbviewer.ipython.org/urls/git.lumc.nl/humgen/programming-course/raw/master/09%20-%20A%20sip%20of%20Biopython%20(2).ipynb