README.md: added code triage badge link [skip ci]

- codetriage is an app with the goal of getting people involved
in open source projects, sending to subscribers a random
open issue for them to help "triage"
- more at: https://www.codetriage.com
- based on PR #1389

README.md

@@ -1,6 +1,7 @@
 # Kamailio - The Open Source SIP Server
 
 [![Build Status](https://travis-ci.org/kamailio/kamailio.svg?branch=master)](https://travis-ci.org/kamailio/kamailio)
+[![Code Triage Badge](https://www.codetriage.com/kamailio/kamailio/badges/users.svg)](https://www.codetriage.com/kamailio/kamailio)
 
 Project Website: