readme.md: add CI build status badges

readme.md

@@ -1,4 +1,4 @@
-TinyXML-2
+TinyXML-2 [![TravisCI Status](https://travis-ci.org/leethomason/tinyxml2.svg?branch=master)](https://travis-ci.org/leethomason/tinyxml2) [![AppVeyor Status](https://ci.appveyor.com/api/projects/status/github/leethomason/tinyxml2?branch=master&svg=true)](https://ci.appveyor.com/project/leethomason/tinyxml2)
 =========
 ![TinyXML-2 Logo](http://www.grinninglizard.com/tinyxml2/TinyXML2_small.png)