Create PR template (omry#1119)

Co-authored-by: Omry Yadan <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thank you for sending a PR and taking the time to improve OmegaConf -->
+
+## Motivation
+
+(Write your motivation for proposed changes here.)
+
+### Have you read the [Contributing Guidelines on pull requests](https://github.com/omry/omegaconf/blob/master/CONTRIBUTING.md)?
+
+Yes/No
+
+## Test Plan
+
+(How should this PR be tested? Do you require special setup to run the test or repro the fixed bug?)
+
+## Fixes
+
+What issue does this PR fix? Use https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue to link this PR to a corresponding issue.
+
+Fixes #<issue_number>
+
+## Related PRs
+
+(Is this PR part of a group of changes? Link the other relevant PRs.)