Add issue template

ISSUE_TEMPLATE.md

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+**System and Shadowsocksx-NG version:**
+
+- macOS 10.12.4 beta (16E144f)
+- Shadowsocksx-NG 1.4
+
+**Expected behavior:**
+
+
+
+**Actual behavior:**
+
+(N/A for feature requests)
+
+**Steps to reproduce:**
+
+(N/A for feature requests)
+
+**How often does this happen?**
+
+(N/A for feature requests)
+
+**ss-local.log**
+
+Please upload the ss-local.log file here the file is in `~/Library/Logs`
+1) Open 'Advanced Settings -> enable Verbose Mode'
+2) Continue run `Shadowsocksx-NG` for 5 minutes
+3) Upload the `~/Library/Logs/ss-local.log` here (with or without compress)
+
+**Application log**
+
+Open the `Console.app` and search `Shadowsocksx-NG`
+Copy paste the log here
+
+**Crash Log**
+
+If the app crashes and pop up a crash log, please copy and paste here
+