chore: add checklist to PULL_REQUEST_TEMPLATE (hexojs#1022)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,20 @@
-- [ ] Read the [theme publishing doc](https://hexo.io/docs/themes#Publishing) or [plugin publishing doc](https://hexo.io/docs/plugins#Publishing).
+## Check List
+
+**Please read and check followings before submitting a PR.**
+
+- [ ] I want to publish my theme on Hexo official website.
+    - [ ] I have read the [theme publishing doc](https://hexo.io/docs/themes#Publishing).
+    - [ ] `link` URL is correct.
+    - [ ] `preview` URL is correct.
+    - [ ] `preview` URL web site is rendered correctly.
+    - [ ] Add a screenshot to `source/themes/screenshots`.
+    - [ ] Screenshot filename is same as value of `name`.
+    - [ ] Screenshot size is `800 * 500`.
+    - [ ] Screenshot file format is `png`.
+- [ ] I want to publish my plugin on Hexo official website.
+    - [ ] I have read the [plugin publishing doc](https://hexo.io/docs/plugins#Publishing).
+    - [ ] `link` URL is correct.
+- [ ] Others (Update, fix, translation, etc...)
 
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