reference updated.

README.rst

@@ -129,7 +129,7 @@ References
 
 .. [1] M. Selig et al., "NIFTY - Numerical Information Field Theory - a
     versatile Python library for signal inference", submitted to IEEE, 2013;
-    `arXiv:XXXX.XXXX <http://www.arxiv.org/abs/XXXX.XXXX>`_
+    `arXiv:1301.4499 <http://www.arxiv.org/abs/1301.4499>`_
 
 Release Notes
 -------------

nifty_core.py

@@ -108,7 +108,7 @@
     ----------
     .. [#] M. Selig et al., "NIFTY -- Numerical Information Field Theory --
         a versatile Python library for signal inference", submitted to IEEE,
-        2013
+        2013; `arXiv:1301.4499 <http://www.arxiv.org/abs/1301.4499>`_
 
 """
 from __future__ import division