Added version numbering information

This adds a section to ``README.rst`` that both specifies the SOFA version this is derived from and outlines the differences.

Note that this conflicts with #7 in that it also adds the "differences from SOFA" section.  Whichever gets merged second will need to be rebased/merged to include this information, but that's straightforward enough.

Also note that once #6 is resolved, the ``x.x`` of the current version should be updated appropriately.