added example image to readme

README.md

@@ -69,4 +69,4 @@ For example, we can statistically show that the prison-punishment was "invented"
 
 Output:
 
-![Image description](https://gitlab.cl.uni-heidelberg.de/born/obc2kg/blob/master/src/imprison-gender-time.png)
+![plot](https://gitlab.cl.uni-heidelberg.de/born/obc2kg/blob/master/src/imprison-gender-time.png)