docs: Add an image showing current Travis build status

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

README.md

 # OP-TEE build.git
+
+<!-- Entire row is copy/pasted from Travis build status URL generation -->
+[![Build Status](https://travis-ci.org/OP-TEE/build.svg?branch=master)](https://travis-ci.org/OP-TEE/build)
+
 ## Contents
 1. [Introduction](#1-introduction)
 2. [Why repo?](#2-why-repo)