Add .mailmap

The mailmap file is used when generating the AUTHORS file
and allows to merge entries by AUTHORS with several email
addresses into one canonical entry.

Added content to .mailmap to merge two addresses of
Ying Chun Guo

Change-Id: I5117ff27a796cf6cdda46f0c3adc3e01e9b88146

.mailmap

@@ -0,0 +1,4 @@
+# Format is:
+# <preferred e-mail> <other e-mail 1>
+# <preferred e-mail> <other e-mail 2>
+<guoyingc@cn.ibm.com> <daisy.ycguo@gmail.com>