Added build-passing image.

README.md

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 This is the working area for the IETF [CORE Working Group](https://datatracker.ietf.org/wg/core/documents/) Internet-Draft, "Group Communication for the Constrained Application Protocol (CoAP)".
 
-* [Editor's Copy](https://core-wg.github.io/groupcomm-bis/#go.draft-ietf-core-groupcomm-bis.html)
+* [Editor's Copy](https://core-wg.github.io/groupcomm-bis/#go.draft-ietf-core-groupcomm-bis.html) [![Build Status](https://travis-ci.org/core-wg/groupcomm-bis.svg?branch=master)](https://travis-ci.org/core-wg/groupcomm-bis)
 * [Working Group Draft](https://tools.ietf.org/html/draft-ietf-core-groupcomm-bis)
 * [Compare Editor's Copy to Working Group Draft](https://core-wg.github.io/groupcomm-bis/#go.draft-ietf-core-groupcomm-bis.diff)