Create pull_request_template.md (#3276)

[skip ci]

.github/pull_request_template.md

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+<!--
+
+Thanks for your contribution!
+
+Please check the following items: (not mandatory)
+
+- Adjacent rules should be sorted alphabetically
+- It's not encouraged to create new list/file containing too few (one or two) rules
+- Newly added list should be included in relevant categories if possible
+- Subdomains are unnecessary as they are overridden by the parent domain. e.g. `[domain:]example.com` overrides `[domain:]app.example.com`
+- Description for your changes is welcome (why the change is necessary, data source of added domains, potential impacts, etc.)
+
+-->
+