Added PR template

.github/pull_request_template.md

@@ -0,0 +1,35 @@
+<!--
+Thanks for the Pull Request!
+
+Please review [Contribution Guide](https://github.com/CesiumGS/cesium/blob/main/CONTRIBUTING.md) before opening your first Pull Request.
+
+To ensure your Pull Request is reviewed and accepted quickly, please refer to our [Pull Request Guidelines](https://github.com/CesiumGS/cesium/blob/main/CONTRIBUTING.md#pull-request-guidelines).
+
+--->
+
+# Description
+
+<!--- Describe your changes in detail -->
+
+<!--- Consider: Why is this change required? What problem does it solve? -->
+
+<!--- Include screenshots if appropriate --->
+
+## Issue number and link
+
+<!--- If it fixes an open issue, link to the issue here -->
+
+<!--- Consider: If suggesting a new feature or change, discuss it in an issue first. -->
+
+## Testing plan
+
+<!--- Describe in detail how you tested your changes -->
+
+# Author checklist
+
+- [ ] I have submitted a Contributor License Agreement
+- [ ] I have added my name to `CONTRIBUTORS.md`
+- [ ] I have updated `CHANGES.md` with a short summary of my change
+- [ ] I have added or updated unit tests to ensure consistant code coverage
+- [ ] I have update the inline documentation, and included code examples where relevant
+- [ ] I have performed a self-review of my code