Update and reorganize Python docs

doc/src/Python_ext.rst

@@ -0,0 +1,16 @@
+Extending the library and Python interface
+******************************************
+
+As noted above, these Python class methods correspond one-to-one with
+the functions in the LAMMPS library interface in src/library.cpp and
+library.h.  This means you can extend the Python wrapper via the
+following steps:
+
+* Add a new interface function to src/library.cpp and
+  src/library.h.
+* Rebuild LAMMPS as a shared library.
+* Add a wrapper method to python/lammps.py for this interface
+  function.
+* You should now be able to invoke the new interface function from a
+  Python script.
+