igl | Add pull_request_template.md

Reviewed By: rudybear

Differential Revision: D67409547

fbshipit-source-id: 66e9db3c63866699f6b02f918fcce97efd271518

.github/pull_request_template.md

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+Our [contribution guidelines](https://github.com/facebook/igl/blob/main/CONTRIBUTING.md):
+
+## Pull Requests
+We actively welcome your pull requests. Here's the procedure to submit pull requests to IGL:
+
+1. **Submit an issue describing your proposed changes.**
+2. The repo owner will respond to your issue.
+3. If your proposed changes are accepted, fork the repo and develop & test your changes.
+4. If you've added code that should be tested, add tests (and ensure they do pass).
+5. Verify your changes work as expected on all relevant rendering backends, and also test them when any combination of backends is disabled using the `IGL_WITH_*` CMake options.
+6. If you've changed APIs, update the documentation.
+7. Make sure your code lints (run `clang-format` or use `.clang-format` in Visual Studio).
+8. If you haven't already, complete the Contributor License Agreement ("CLA").
+9. Please respect `// @fb-only` comments and do not delete them!
+10. Open a pull request.
+
+## Dependencies
+One of our design objectives is that IGL itself should have as few dependencies as possible.
+No new third-party dependencies will be accepted unless they are absolutely critical to the core functionality of IGL.
+
+**Please include a link to the related (and accepted) GitHub issue in this PR!**