More edits for including logo.

README.rst

@@ -2,9 +2,8 @@
     :target: http://docs.gprmax.com/en/latest/?badge=latest
     :alt: Documentation Status
 |
-.. raw:: html
-
-    <a href="http://www.gprmax.com"><img src="https://github.com/gprMax/gprMax/blob/master/docs/source/images/gprMax_logo.png" height="150px" width="525px"></a>
+.. image:: docs/source/images/gprMax_logo_small.png
+    :target: http://www.gprmax.com
 
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 Getting Started