Add design docs and PR templates

Author: polldo

Diff for: designs/_pull_request_template.md

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+# Pull Request Template
+
+## Title
+
+```
+[Design] title of the design document
+```
+
+## Description
+
+```
+### Reviewer workflow
+
+- [ ] First draft (problem statement, pick a candidate solution).
+- [ ] Edited and discussed.
+- [ ] Reviewed by peers.
+- [ ] Reviewed by stakeholders.
+```

Diff for: designs/_template.md

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+# Title: Template for design documents
+
+Use this as a template to write a design document when adding new
+major features to the project. It helps other developers
+understand the scope of the project, validate technical complexity and
+feasibility. It also serves as a public documentation of how the feature
+actually works.
+
+## Review Period
+
+Best before Month, day year.
+
+## What is the problem?
+
+## Out-of-Scope
+
+## User Experience Walkthrough
+
+## Implementation
+
+### Project Changes
+
+_Explain the changes to api or command line interface, including adding new
+commands, modifying arguments etc_
+
+### Breaking Change
+
+_Are there any breaking changes to the interface? Explain_
+
+### Design
+
+_Explain how this feature will be implemented. Highlight the components
+of your implementation, relationships_ _between components, constraints,
+etc._
+
+### Security
+
+_Tip: How does this change impact security? Answer the following
+questions to help answer this question better:_
+
+**What new dependencies (libraries/cli) does this change require?**
+
+**What other Docker container images are you using?**
+
+**Are you creating a new HTTP endpoint? If so explain how it will be
+created & used**
+
+**Are you connecting to a remote API? If so explain how is this
+connection secured**
+
+**Are you reading/writing to a temporary folder? If so, what is this
+used for and when do you clean up?**
+
+### Documentation Changes
+
+_Explain the changes required to internal and public documentation (API reference, tutorial, etc)_
+
+## Additional Notes
+
+_Link any useful metadata: Jira task, GitHub issue, …_