Create Pull Request Template

Fixes #46

Author: pawel-lewtak

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- 
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+* *Fixed a typo in the apikeypermissions.md page*
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+**Description of the change**:
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+**Reason for the change**:
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+**Link to original source**:
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+### Checklist
+
+Make sure all of these items are complete, or else the PR will be ineligible for a code review.
+
+- [ ] Code passes all existing [tests](https://github.com/sendgrid/php-http-client/tree/master/test)
+- [ ] Any new functionality added includes new unit tests in [`tests/unit`](https://github.com/sendgrid/php-http-client/tree/master/test/unit)
+- [ ] Create or update example code to show the new functionality in action.
+- [ ] All code, branch, and git naming and style conventions are followed (see [`CONTRIBUTING.md`](https://github.com/sendgrid/php-http-client/blob/master/CONTRIBUTING.md#style-guidelines-and-naming-conventions))
+- [ ] Feature branch has been rebased off of the latest `master` branch. (see [`CONTRIBUTING.md`](https://github.com/sendgrid/php-http-client/blob/master/CONTRIBUTING.md#creating-a-pull-request)).
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+If you have questions, please send an email [Sendgrid](mailto:[email protected]), or file a Github Issue in this repository.
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+<!-- 
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