Create issue_template.md (#932)

* Create issue_template.md

* Update issue_template.md


Former-commit-id: 207a121296d4301afcd23759e38fea5f15c065b2

.github/issue_template.md

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+[Describe your issue]
+
+#### Check all that apply (change to `[x]`)
+- [ ] Windows
+- [ ] Mac OS X
+- [ ] Linux
+
+See <https://libigl.github.io/CONTRIBUTING/#bugreport> for more tips.