issue template

.github/issue_template.md

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+#### Check all platforms you experienced the issue (change to `[x]`)
+- [ ] Windows
+- [ ] Mac OS
+- [ ] Linux
+- [ ] Every platform
+
+#### What type of issue is?
+- [ ] bug or unexpected behavior;
+- [ ] crash;
+- [ ] feature request;
+- [ ] other (specify...)
+
+#### Describe your issue
+
+...
+
+####Guidelines for a good issue
+
+Please before submitting an issue, check if your issue has been already reported by someone else. In this case, do not open another issue: just add your experience in the existing issue.
+
+#####Subject
+If the issue is specific of a platform, please specify it on the subject in square brackets (example: `[MacOS] crash when opening obj`). 
+Please specify also if the issue concerns a bug, crash, or it is a feature request.
+
+#####Description
+Please be as detailed as possible. If the issue is related to a specific mesh or file, please attach it. Delete these guidelines before submitting :)