Add .mailmap file

This adds a .mailmap file that maps old users.sourceforge.net e-mail
addresses (mostly) for some contributors to the current e-mail addresses
with which they’ve committed to GitHub repos.

The effect of this is to make GitHub recognize the users such that they
show up in Contributors graph for the xslt10-stylesheets repo at
https://github.com/docbook/xslt10-stylesheets/graphs/contributors

.mailmap

@@ -0,0 +1,8 @@
+Adam Di Carlo <[email protected]> <[email protected]>
+Cedric Pronzato <[email protected]> <[email protected]>
+Gábor Kövesdán <[email protected]> <[email protected]>
+Keith Fahlgren <[email protected]> <[email protected]>
+Martijn van Beers <[email protected]> <[email protected]>
+Michael[tm] Smith <[email protected]> <[email protected]>
+mzjn <[email protected]> <[email protected]>
+Steve Ball <[email protected]> <[email protected]>