Add Olivier Mengué to the list of contributors.

Thanks for the MSysGit installer!
diff --git a/AUTHORS b/AUTHORS
index 69e9b13..d4e7490 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -15,5 +15,6 @@
 - Konstantin Tjuterev
 - Kiall Mac Innes
 - Jon Bernard
+- Olivier Mengué
 
 Portions derived from other open source works are clearly marked.