[skip ci] MEP template (#6366)

Signed-off-by: yefu.chen <yefu.chen@zilliz.com>

docs/design_docs/MEP-Template.md

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+# MEP(Milvus Enhancement Proposals) Templates
+Current state: [One of "Under Discussion", "Accepted", "Rejected"]
+
+ISSUE: link to the github issue
+
+PRs: link to the github PRs
+
+Keywords: list keywords about this mep
+
+Released: <Milvus Release Version>
+
+## Summary(required)
+
+What are we going to do?
+
+## Motivation(required)
+
+Why are we doing this?
+
+## Public Interfaces(optional)
+
+Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed.
+
+## Design Details(required)
+
+Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.
+
+## Compatibility, Deprecation, and Migration Plan(optional)
+- What impact (if any) will there be on existing users?
+- If we are changing behavior how will we phase out the older behavior?
+- If we need special migration tools, describe them here.
+- When will we remove the existing behavior?
+
+
+## Test Plan(required)
+
+Describe in few sentences how the MEP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?
+
+## Rejected Alternatives(optional)
+
+If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.
+
+## References(optional)
+
+Briefly list all references
+
+