default issue template

.github/ISSUE_TEMPLATE.md

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+如果你遇到的问题不是 V2Ray 的 bug，比如你不清楚要如何配置，请使用[Discussion](https://github.com/v2ray/discussion/issues)进行讨论。
+
+此 Issue 会被立即关闭。
+
+If you are not sure if your question is truely a bug in V2Ray, please discuss it [here](https://github.com/v2ray/discussion/issues) first.
+
+This issue will be closed immediately.