Docs: add KEPs introduction to the main repo (#5099)

Signed-off-by: Somefive <yd219913@alibaba-inc.com>

Signed-off-by: Somefive <yd219913@alibaba-inc.com>

design/README.md

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+# KubeVela Enhancement Proposals (KEPs)
+
+This directory is a place to propose and discuss new ideas of KubeVela concepts, designs, architectures and techniques.
+
+### When do we need KEPs
+
+When major changes are intended to be made to KubeVela project, we need KEPs. Major changes includes:
+- New project-level features that add modules to the architecture, like new Controller or APIServer.
+- Break changes to the core concepts of KubeVela, such as Application, Workflow, Component, etc.
+- Techniques or domains that lots of related enhancements need to be added to KubeVela, like multi-cluster, observability, etc.
+
+Changes to the internal mechanism of core KubeVela are recommended to add proposals as well, including:
+- Application behaviours and related policies: State-keep, garbage-collect, resource dispatch, etc.
+- API changes of auxiliary resources in KubeVela, such as ApplicationRevision, ResourceTracker.
+- New concepts and layers in KubeVela APIServer on VelaUX, such as Project, Target, etc.
+
+Minor changes and enhancements do not necessarily need to be included, but instead recommended to be tracked by issues, such as 
+- New addons.
+- New Component/Trait/WorkflowStep definitions.
+- New additional function APIs in APIServer.
+- Bug detection and fixes.
+- Auxiliary commands in CLI.
+
+### Areas
+
+There are several directories contained. Each directory contains the KEPs in specific area.
+
+- **/vela-core**: The proposals of features and changes to the core KubeVela, including Application APIs, internal mechanisms, auxiliary policies, etc.
+- **/vela-cli**: The proposals of features to the KubeVela CLI, such as `vela top`, `vela def`.
+- **/api**: The proposals of the interfaces KubeVela exposes to users, such as command line args for the core controller.
+- **/platform**: The proposals of integrating features in various related areas outside KubeVela, such as edge computing, artificial intelligence.
+- **/resources**: The related images embedded in the design documentations.
+
+### Writing a new Proposal
+
+The aim of a proposal is to communicate designs with others and give KubeVela users some basic ideas of how features and evolved and developed.
+
+To reach that, there are several things seed to be included in a proposal.
+1. The background of the feature or change, which explains why we need it.
+2. The goals and non-goals for the proposal.
+3. The potential technical solutions for the proposal and comparisons between various solutions. (Single solution is also acceptable.)
+4. How we should move on for the proposal. The estimated milestones or timelines for the feature development.
+
+### Submitting a new proposal
+
+We recommend to use the [template](./TEMPLATE.md) to start a new proposal.
+After finishing the proposal in the proper directory, raise a pull request to add the proposal to the main repo.
+If there are any issues related to the proposal, you can also add links to the issues in the pull request.

design/TEMPLATE.md

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+# NAME OF THE PROPOSAL
+
+### (Optional) Introduction
+
+A short introduction to the proposal.
+
+### Background
+
+Why we need the proposal.
+
+### (Optional) Goals & Non-Goals
+
+The goals and non-goals of the proposal.
+
+### Proposal
+
+The potential solutions and comparisons. Technique details should be placed here. Risks for break changes should be highlighted.
+
+### (Optional) Examples
+
+The examples of how to use if the proposal is completed.
+
+### (Optional) Progress / Timeline / Milestones
+
+Break the proposal implementation into atomic units and estimate the time cost and difficulties for the implementation.