Create pull_request_template.md

Adds a pull request template in order to enhance pull request descriptions.

.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. -->
+
+## 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 which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to change)
+
+## 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 have checked that there aren't other open Pull Requests for the same update/change.
+- [ ] My code follows the code style of this project.
+- [ ] I have performed a self-review of my own code.
+- [ ] I have commented my code, particularly in hard-to-understand areas.
+- [ ] \(Optional) My change requires a change to the documentation.
+- [ ] \(Optional) I have updated the documentation accordingly.
+- [ ] \(Optional) My change requires a change to the translations.
+- [ ] \(Optional) I have updated the translations accordingly.
+- [ ] `composer cs:check` has passed locally.
+- [ ] `composer lint:php` has passed locally.
+- [ ] I have read the **CONTRIBUTING** document.
+
+## Further comments
+<!--- If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc... -->