Add enhancements document template

Heavily borrowed from Kubernetes KEP template.

Signed-off-by: Sheng Yang <sheng.yang@rancher.com>

enhancements/YYYYMMDD-template.md

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+# Title
+
+This is the title of the enhancement. Keep it simple and descriptive.
+
+The file name should be lowercased and spaces/punctuation should be replaced with `-`.
+
+## Summary
+
+The Summary section is incredibly important for producing high quality user-focused documentation such as release notes or a development roadmap.
+
+A good summary is probably at least a paragraph in length.
+
+### Related PR
+
+The URL For the related enhancement PR in Longhorn repo.
+
+## Motivation
+
+### Goals
+
+List the specific goals of the enhancement. How will we know that this has succeeded?
+
+### Non-goals [optional]
+
+What is out of scope for this enhancement? Listing non-goals helps to focus discussion and make progress.
+
+## Proposal
+
+This is where we get down to the nitty gritty of what the proposal actually is.
+
+### User Stories [optional]
+Detail the things that people will be able to do if this enhancement is implemented. Include as much detail as possible so that people can understand the "how" of the system. The goal here is to make this feel real for users without getting bogged down.
+
+#### Story 1
+#### Story 2
+
+### User experience description
+
+Detail what user need to do to use this enhancement.
+
+## Design
+
+### Implementation Overview
+
+Overview on how the enhancement will be implemented.
+
+### Test plan
+
+Integration test plan.
+
+For engine enhancement, also requires engine integration test plan.
+
+### Upgrade strategy
+
+Anything that requires if user want to upgrade to this enhancement
+
+## Note [optional]
+
+Additional nodes.