Added pull request template (#793)

* Added pull request template
Signed-off-by: Przemek Tredak <ptredak@nvidia.com>

* Changes from the review
Signed-off-by: Przemek Tredak <ptredak@nvidia.com>

---------
Signed-off-by: Przemek Tredak <ptredak@nvidia.com>

.github/PULL_REQUEST_TEMPLATE.md

+# Description
+
+Please include a brief summary of the changes, relevant motivation and context.
+
+Fixes # (issue)
+
+## Type of change
+
+- [ ] Documentation change (change only to the documentation, either a fix or a new content)
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
+
+## Changes
+
+Please list the changes introduced in this PR:
+
+- Change A
+- Change B
+
+# Checklist:
+
+- [ ] I have read and followed the [contributing guidelines](https://github.com/NVIDIA/TransformerEngine/blob/main/CONTRIBUTING.rst)
+- [ ] The functionality is complete
+- [ ] I have commented my code, particularly in hard-to-understand areas
+- [ ] I have made corresponding changes to the documentation
+- [ ] My changes generate no new warnings
+- [ ] I have added tests that prove my fix is effective or that my feature works
+- [ ] New and existing unit tests pass locally with my changes