Added design doc template

design_documents/template.md

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+# <TITLE>
+
+## Rationale of the solution 
+Provide use cases of the feature and general motivation why it is worth implementing/was implemented. You can also include some 
+existing solutions.
+
+## User perspective description of the solution
+Provide only if documentation doesn't already exist and if the feature is user-facing. 
+Provide an overview of the feature API from the user perspective. Consider it a draft documentation which you can later reuse as documentation.
+
+## Technical overview of the solution
+Provide a rough description of the feature implementation. Keep it high level. Try to include your mental model if possible.
+Try to include motivation behind non-obvious technical decisions. This is important so we don't have to rethink details and motivations it every time.
+If something is an open question include it as well.
+
+
+## Implementation plan
+Optional - provide only if feature is not implemented and you have some steps in mind which you will take. Keep it high level!
+