Create issue_template.md

Author: ValentinH

issue_template.md

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+### Steps to reproduce
+1. 
+2. 
+3. 
+
+Demo: http://jsfiddle.net/cwhgLcjv/ (fork this example and update the link)
+
+### Expected behaviour
+Tell us what should happen
+
+### Actual behaviour
+Tell us what happens instead