STYLE: Add icon passed/failed in README linked to CDash

Collect the result of the continuous integration of a commit pushed and display if the changes have passed or failed the configuration/build/test

README

@@ -1,3 +1,9 @@
+Common Toolkit
+==============
+
+.. image:: https://circleci.com/gh/commontk/CTK.svg?style=svg
+    :target: http://my.cdash.org/index.php?project=CTK
+
 The Common Toolkit is a community effort to provide support code for medical image analysis,
 surgical navigation, and related projects.