repo: add HIP template

This template was sourced from https://github.com/chrisabruce/templates

0000-template.md

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+- Start Date: <!-- fill me in with today's date, YYYY-MM-DD -->
+- HIP PR: <!-- leave this empty -->
+- Tracking Issue: <!-- leave this empty -->
+
+# Summary
+[summary]: #summary
+
+One paragraph explanation of the proposal.
+
+# Motivation
+[motivation]: #motivation
+
+Why are we doing this? What use cases does it support? What problems does it
+solve? What is the expected outcome?
+
+# Stakeholders
+[stakeholders]: #stakeholders
+
+* Who is affected by this HIP?
+
+* How are we soliciting feedback on this HIP from these stakeholders? Note that
+ they may not be watching the HIPs repository or even aren't directly active in
+ the Rust Async Ecosystem working group.
+
+# Detailed Explanation
+[detailed-explanation]: #detailed-explanation
+
+- Introduce and explain new concepts.
+
+- It should be reasonably clear how the proposal would be implemented.
+
+- Provide representative examples that show how this proposal would be commonly
+ used.
+
+- Corner cases should be dissected by example.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- Why should we *not* do this?
+
+# Rationale and Alternatives
+[alternatives]: #rationale-and-alternatives
+
+This is your chance to discuss your proposal in the context of the whole design
+space. This is probably the most important section!
+
+- Why is this design the best in the space of possible designs?
+
+- What other designs have been considered and what is the rationale for not
+ choosing them?
+
+- What is the impact of not doing this?
+
+# Unresolved Questions
+[unresolved]: #unresolved-questions
+
+- What parts of the design do you expect to resolve through the HIP process
+ before this gets merged?
+
+- What parts of the design do you expect to resolve through the implementation
+ of this feature?
+
+- What related issues do you consider out of scope for this HIP that could be
+ addressed in the future independently of the solution that comes out of this
+ HIP?
+
+# Deployment Impact
+[deployment-impact]: #deployment-impact
+
+Describe how this design will be deployed and any potential imapact it may have on
+current users of this project.
+
+- How will current users be impacted?
+
+- How will existing documentation/knowlegebase need to be supported?
+
+- Is this backwards compatible?
+
+  - If not, what is the procedure to migrate?
+
+# Success Metrics
+[success-metrics]: #success-metrics
+
+What metrics can be used to measure the success of this design?
+
+- What should we measure to prove a performance increase?
+
+- What should we measure to prove an improvement in stability?
+
+- What should we measure to prove a reduction in complexity?
+
+- What should we measure to prove an acceptance of this by it's users?