docs: add simple pull request template

.github/pull_request_template.md

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+<!--- Provide a general summary of your changes in the Title above -->
+
+## Description
+<!--- Describe your changes in detail -->
+
+## Motivation and Context
+<!--- Why is this change required? What problem does it solve? -->
+<!--- If it fixes an open issue, please link to the issue here. -->
+
+## How Has This Been Tested?
+<!--- Please describe in detail how you tested your changes. -->
+<!--- Include details of your testing environment, the tests you ran to -->
+<!--- see how your change affects other areas of the code, etc. -->
+
+## Screenshots (if appropriate):
+
+## Types of Changes
+<!--- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
+- [ ] Bug fix (non-breaking change that fixes an issue)
+- [ ] New feature (non-breaking change that adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
+
+## Checklist:
+<!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
+<!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
+- [ ] I followed the [Contributing Guidelines](https://github.com/Soundux/Soundux/blob/master/CONTRIBUTING.md#code) (including code style) of this project.
+- [ ] My change requires a change to the documentation.