Use the URL of the ReFrame logo in README.md

Author: Theofilos Manitaras

README.md

@@ -1,5 +1,5 @@
 <a href="http://github.com/eth-cscs/reframe">
-  <img src="docs/_static/img/reframe_logo-full.png" width=400>
+  <img src="https://github.com/eth-cscs/reframe/blob/master/docs/_static/img/reframe_logo-full.png" width=400>
 </a>
 
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