explain how to make a proper bug report (#1579)

fixes #1574

Author: alexlamsl

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,7 @@
+- Bug report or feature request?
+- [ ] `uglify-js` version (`uglifyjs -V`)
+- [ ] JavaScript input - ideally as small as possible.
+- [ ] The `uglifyjs` CLI command executed or `minify()` options used.
+- [ ] An example of JavaScript output produced and/or the error or warning.
+
+Note: the release version of `uglify-js` only supports ES5. Those wishing to minify ES6 should use the experimental [`harmony`](https://github.com/mishoo/UglifyJS2#harmony) branch.