chore: add pull request template (#417) [skip ci]

* Add pull_request_template.md

* Improve pull request template

* Minor edit

.github/pull_request_template.md

@@ -0,0 +1,16 @@
+After you have read the [instructions for adding a theme](https://github.com/gohugoio/hugoThemesSiteBuilder/blob/main/README.md#adding-a-theme), please make sure:
+
+- [ ] your theme.toml [is complete](https://github.com/gohugoio/hugoThemesSiteBuilder/blob/main/README.md#theme-configuration)
+    - [ ] name
+    - [ ] license
+    - [ ] licenselink
+    - [ ] description
+    - [ ] homepage
+    - [ ] demosite (if one exists)
+    - [ ] tags
+    - [ ] features
+    - [ ] authors/author (depending on whether the theme has multiple or a single author)
+    - [ ] original (if the theme is a fork)
+- [ ] you're using [absolute paths for images](https://github.com/gohugoio/hugoThemesSiteBuilder/blob/main/README.md#use-absolute-paths-for-images) in your README
+- [ ] you've got [appropriate thumbnail and screenshot images](https://github.com/gohugoio/hugoThemesSiteBuilder/blob/main/README.md#media)
+- [ ] your theme comes with an [appropriate license](https://github.com/gohugoio/hugoThemesSiteBuilder/blob/main/README.md#2-license)