README.md: Add a reference to midonet.atlassian.net

Change-Id: Icc8a784b5d8c0faef89a6988a9052d4d6f3ee368
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Merge "README.md: Add a reference to midonet.atlassian.net"