Added RTD badge.

README.rst

@@ -3,6 +3,10 @@
 Getting Started
 ***************
 
+.. image:: https://readthedocs.org/projects/gprmax/badge/?version=latest
+    :target: http://docs.gprmax.com/en/latest/?badge=latest
+    :alt: Documentation Status
+
 What is gprMax?
 ===============