Create ISSUE_TEMPLATE.md (#3012)

ISSUE_TEMPLATE.md

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+For feature requests, please see: http://www.mattermost.org/feature-requests/.  
+For troubleshooting questions, please post on the forums: http://forum.mattermost.org/.
+
+For issues with the product, please follow the steps at http://www.mattermost.org/filing-issues/ and post here in the following format: 
+
+#### Summary
+Issue in one concise sentence
+
+#### Steps to reproduce
+How can we reproduce the issue (what version are you using?)
+
+#### Expected behavior
+Describe your issue in detail
+
+#### Observed behavior
+What did you see happen? Please include relevant error messages and/or screenshots.
+
+#### Possible fixes
+If you can, link to the line of code that might be responsible for the problem