GitHub: Add pull request template

.github/pull_request_template.md

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+**Complete description of the bug fix or feature that this pull request implements**
+
+
+**Checklist before submitting the pull request, to maximize the chances that the pull request will be accepted**
+
+- [ ] Read CONTRIBUTING.md, a link to which appears under "Helpful resources" below.  That document discusses general guidelines for contributing to libjpeg-turbo, as well as the types of contributions that will not be accepted or are unlikely to be accepted.
+- [ ] Search the existing issues and pull requests (both open and closed) to ensure that a similar request has not already been submitted and rejected.
+- [ ] Discuss the proposed bug fix or feature in a GitHub issue, through direct e-mail with the project maintainer, or on the libjpeg-turbo-devel mailing list.