Add issue template

ISSUE_TEMPLATE.md

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+Brief description:
+
+Operating System: Windows / OS X / Linux (include version and/or distro)  
+qTox version: (shown on `About` page in settings)  
+Hardware: 
+…
+
+Reproducible: Always / Almost Always / Sometimes / Rarely / Couldn't Reproduce
+
+
+**Steps to reproduce:**  
+1.  
+2.  
+3.  
+…
+
+
+**Observed Behavior:**  
+
+
+**Expected Behavior:**  
+
+
+**Additional info:**  
+(links, images, etc go here)
+
+----
+
+More information on how to write good bug reports in the wiki: https://github.com/tux3/qTox/wiki/Writing-Useful-Bug-Reports