Update ISSUE_TEMPLATE.md (#2576)

.github/ISSUE_TEMPLATE.md

@@ -3,6 +3,7 @@ Please provide the following when reporting an issue:
 - [ ] The version of Turf you are using, and any other relevant versions.
 - [ ] GeoJSON data as a [gist file](https://gist.github.com/) or [geojson.io](http://geojson.io/) (filename extension must be `.geojson`).
 - [ ] Snippet of source code or for complex examples use [jsfiddle](https://jsfiddle.net/).
+- [ ] Verify this issue hasn't already been reported, or resolved in the latest alpha pre-release.
 
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