mailmap: provide usage instruction

Looking at the contents of file .mailmap it seems that some editors assumed
that translation is done by entering multiple lines into the file and the
last one replaces the others. This is not how it works. The translation
occurs according to entries in single lines as described in the
git-check-mailmap man-page.

Add a description of the file format.

Add an entry for Alexander Graf as his old email address is not valid
anymore.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>