Describing the MERGE implementation.
Fix bug in RML/MERGE2.  Add comments.

RML/MERGE2 was not handling lists with an odd number of elements. As a
result, It was calling CAR on numbers.  This patch adds two checks for
the cases when either LEFT or RIGHT are just numbers (we already had a
check for the case when LEFT and RIGHT were both numbers).
Git, ignore more things.
Convert to ORG.  Fix bug in MERGE.

Added a little more to the intro of the sorting chapter as well.
Trying to explain the architecture of a merge sort implementation
before diving in.