Create PULL_REQUEST_TEMPLATE.md

Thanks @stevemao (https://github.com/stevemao/github-issue-templates)

Author: fpistm

.github/PULL_REQUEST_TEMPLATE.md

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+## Pull Request template
+
+Please, Make sure that your PR is not a duplicate.
+Search among the [Pull request](https://github.com/stm32duino/Arduino_Core_STM32/pulls) before creating one.
+
+IMPORTANT: Please review the [CONTRIBUTING.md](CONTRIBUTING.md) file for detailed contributing guidelines.
+
+Thanks for submitting a pull request.
+Please provide enough information so that others can review your pull request:
+
+**Summary**
+
+<!-- Summary of the PR -->
+
+This PR fixes/implements the following **bugs/features**
+
+* [ ] Bug 1
+* [ ] Bug 2
+* [ ] Feature 1
+* [ ] Feature 2
+* [ ] Breaking changes
+
+<!-- You can skip this if you're fixing a typo or adding an app to the Showcase. -->
+
+Explain the **motivation** for making this change. What existing problem does the pull request solve?
+
+<!-- Example: When "Adding a function to do X", explain why it is necessary to have a way to do X. -->
+
+**Validation**
+
+* Ensure Travis CI build is passed.
+* Demonstrate the code is solid. [e.g. Provide a sketch]
+
+<!-- Make sure tests pass on both Travis CI. -->
+
+**Code formatting**
+
+* Ensure AStyle check is passed thanks Travis CI (soon)
+
+<!-- See the simple style guide. -->
+
+**Closing issues**
+
+<!-- Put `closes #XXXX` in your comment to auto-close the issue that your PR fixes (if such). -->
+Fixes #xxx