[Feature] Add PULL_REQUEST_TEMPLATE

[Radeity]

Feature Description (功能描述)

Add PULL_REQUEST_TEMPLATE for this repo.

[Radeity]

I will try to submit a fundamental template tonight, plz assign me : ) cc @coderzc @imbajin

[imbajin]

All repo could use one(similar) template, and could list the template reference & "Actual Effect UI" here

[Radeity]

"Actual Effect UI"

Hi, @imbajin , i've already linked a PR with the new pull request template.

the template reference

All references are as below:

• https://github.com/apache/spark/blob/master/.github/PULL_REQUEST_TEMPLATE
• https://github.com/apache/flink/blob/master/.github/PULL_REQUEST_TEMPLATE.md
• https://github.com/apache/skywalking/blob/master/.github/PULL_REQUEST_TEMPLATE