Added Travis CI to readme

Author: YetAnotherMatt

README.md

@@ -2,6 +2,8 @@
 A builder annotation for Kotlin interoperability with Java.
 This project aims to be a minimal viable replacement for the Lombok @Builder plugin for Kotlin code.
 
+[![Build Status](https://travis-ci.com/ThinkingLogic/kotlin-builder-annotation.svg?branch=master)](https://travis-ci.com/ThinkingLogic/kotlin-builder-annotation.svg?branch=master)
+
 Named constructor parameters with optional values mean that Kotlin code doesn't require the builder pattern any more,
 _unless_ you're writing code that will be used by Java projects 
 (else you consign your java clients to using unhelpful constructors with many parameters).