docs: add issue template

ISSUE_TEMPLATE.md

@@ -0,0 +1,26 @@
+<!--
+Thank you for your contribution.
+
+When it comes to write an issue, please, use the template below.
+To use the template is mandatory for submit new issue and we won't reply the issue that without the template.
+
+And you can write template's contents in Korean also.
+-->
+
+<!-- TEMPLATE -->
+
+## Version
+<!-- Write the version of the tui-editor you are currently using. -->
+
+## Development Environment
+<!-- Write the browser type, OS and so on -->
+
+## Current Behavior
+<!-- Write a description of the current operation. You can add sample code, 'CodePen' or 'jsfiddle' links. -->
+
+```js
+// Write example code
+```
+
+## Expected Behavior
+<!-- Write a description of the future action. -->