The two files are not interchangeable but a change to one *might*
require a change to the other so lets flag that up with an explanation
of what both files are trying to achieve. While we are at it document
the many forms .mailmap can take in the header.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>