Add Travis build status badge

Author: asbjornu

README.md

@@ -2,7 +2,7 @@
 
 A JSON-LD processor for .NET.
 
-[![NuGet][nuget-badge]][nuget] [![codecov][codecov-badge]][codecov]
+[![NuGet][nuget-badge]][nuget] [![Build Status][travis-badge]][travis] [![codecov][codecov-badge]][codecov]
 
 This project has adopted the [Microsoft Open Source Code of Conduct][coc].
 For more information see the [Code of Conduct FAQ][coc-faq] or contact
@@ -40,4 +40,6 @@ This project began life as a [Sharpen][sharpen]-based auto-port from
   [dnc]:            https://dot.net
   [dnc-tutorial]:   https://www.microsoft.com/net/core
   [codecov]:        https://codecov.io/gh/linked-data-dotnet/json-ld.net
-  [codecov-badge]:  https://img.shields.io/codecov/c/github/linked-data-dotnet/json-ld.net/master.svg
+  [codecov-badge]:  https://img.shields.io/codecov/c/github/linked-data-dotnet/json-ld.net/master.svg
+  [travis]:         https://travis-ci.org/linked-data-dotnet/json-ld.net
+  [travis-badge]:   https://img.shields.io/travis/linked-data-dotnet/json-ld.net.svg