Add issue template (react-navigation#1199)

* Add issue template

* Ask for screenshot

* Update ISSUE_TEMPLATE.md

Author: grabbou

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,22 @@
+<!---
+BEFORE YOU SUBMIT please search open/closed issues since someone might have asked the same thing before!
+-->
+
+### Current Behavior
+<!--- If describing a bug, tell us what happens instead of the expected behavior.
+      Provide a screenshot when possible and full code sample -->
+<!--- If suggesting a change/improvement, explain the difference from current behavior -->
+
+### Expected Behavior
+<!--- If you're describing a bug, tell us what should happen -->
+<!--- If you're suggesting a change/improvement, tell us how it should work -->
+
+### Your Environment
+<!--- Include as many relevant details about the environment you experienced the bug in -->
+
+| software         | version
+| ---------------- | -------
+| react-navigation |  
+| react-native     |  
+| node             | 
+| npm or yarn      |