Create PULL_REQUEST_TEMPLATE.md (#4066)

Author: kurtisvg

.github/PULL_REQUEST_TEMPLATE.md

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+## Description
+
+Fixes #<ISSUE-NUMBER>
+
+Note: It's a good idea to open an issue first for discussion.
+
+## Checklist
+- [ ] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
+- [ ] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
+- [ ] **Tests** pass:   `nox -s py-3.6` (see [Test Enviroment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
+- [ ] **Lint** pass:   `nox -s lint` (see [Test Enviroment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
+- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
+- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
+- [ ] Please **merge** this PR for me once it is approved.