DOC: Add .mailmap

Ensure that Tim's last name is consistent no matter how the patch is generated
and applied, preventing Tim from showing up as two different persons in the
shortlog in releases.
diff --git a/.mailmap b/.mailmap
new file mode 100644
index 0000000..30353d7
--- /dev/null
+++ b/.mailmap
@@ -0,0 +1 @@
+Tim Düsterhus <tim@bastelstu.be>