Created Basic Issue Template

inspired by webpack's issue template

ISSUE_TEMPLATE.md

+<!-- Please don't delete this template or we'll close your issue -->
+<!-- Before creating an issue please make sure you are using the latest version of AR.js. -->
+<!-- Before creating an issue please make a search on open and closed issues, maybe your question is already answered. -->
+**Do you want to request a *feature* or report a *bug*?**
+
+<!-- Please ask questions on StackOverflow or the AR.js Gitter (https://gitter.im/AR-js). -->
+<!-- Issues which contain questions or support requests will be closed. -->
+
+**What is the current behavior?**
+
+**If the current behavior is a bug, please provide the steps to reproduce.**
+
+<!-- A great way to do this is to provide an external url, (like a gist, codepen), with a short description on the issue you experiencing -->
+<!-- Check if the issue can be reproduced in one of our demos/examples -->
+<!-- Make sure your issue is not originating from third party source ex: (from A-Frame, Tango, JSARToolkit) -->
+
+**What is the expected behavior?**
+
+**If this is a feature request, what is motivation or use case for changing the behavior?**
+
+**Please mention other relevant information such as the browser version, Operating System and Device Name**