add readme info for tools

Author: jakevdp

tools/README.md

@@ -0,0 +1,7 @@
+# Tools
+
+These are tools for managing the notebooks in this repository.
+
+- ``generate_contents.py``: this will generate a markdown table of contents for use in the README and in the Index.ipynb notebook
+
+- ``add_navigation.py``: this script adds navigation links at the top and bottom of each notebook.