Add design proposal guidelines (#417)

design-proposals/2022-09-02-design-proposal-template.md

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+# Design Proposal Template
+
+## Motivation
+
+Describe the problem you would like to solve.
+
+## Proposed Change
+
+Describe what you would like to do to solve the problem. This part is the actual design proposal and can be extensive with multiple subsections.
+
+## API Modifications
+
+Outline what modifications you expect on the public API due to your change.
+
+## Migration Plan and Compatibility
+
+If your change is incompatible with existing APIs, draft a migration plan to help users adapting to the new version once your change is completed.
+
+## Rejected Alternatives
+
+Discuss what alternatives to your proposed change you considered and why you rejected them.