Add pull request template (#1372)

* Add pull request template

* update pull

.github/PULL_REQUEST_TEMPLATE.md

+## *Tips*
+- *Thanks very much for contributing to Apache DolphinScheduler.*
+- *Please review https://dolphinscheduler.apache.org/en-us/community/index.html before opening a pull request.*
+
+## What is the purpose of the pull request
+
+*(For example: This pull request adds checkstyle plugin.)*
+
+## Brief change log
+
+*(for example:)*
+  - *Add maven-checkstyle-plugin to root pom.xml*
+
+## Verify this pull request
+
+*(Please pick either of the following options)*
+
+This pull request is code cleanup without any test coverage.
+
+*(or)*
+
+This pull request is already covered by existing tests, such as *(please describe tests)*.
+
+(or)
+
+This change added tests and can be verified as follows:
+
+*(example:)*
+
+  - *Added dolphinscheduler-dao tests for end-to-end.*
+  - *Added CronUtilsTest to verify the change.*
+  - *Manually verified the change by testing locally.*