added Zenodo DOI badge to citation page

Toby Hodges · Toby Hodges · commit 72a83124f978 · 2017-08-08T16:06:16.000+02:00
diff --git a/CITATION b/CITATION
@@ -1 +1,3 @@
-FIXME: describe how to cite this lesson.
+# Citation
+
+[![DOI](https://zenodo.org/badge/89621457.svg)](https://zenodo.org/badge/latestdoi/89621457)
