Add upgrading instructions

Signed-off-by: RainbowMango <qdurenhongcai@gmail.com>

docs/upgrading/README.md

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+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+**Table of Contents**  *generated with [DocToc](https://github.com/thlorenz/doctoc)*
+
+- [Upgrading Instruction](#upgrading-instruction)
+  - [Overview](#overview)
+  - [Regular Upgrading Process](#regular-upgrading-process)
+    - [Upgrading APIs](#upgrading-apis)
+      - [Manual Upgrade API](#manual-upgrade-api)
+    - [Upgrading Components](#upgrading-components)
+  - [Details Upgrading Instruction](#details-upgrading-instruction)
+    - [v0.8 to v0.9](#v08-to-v09)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+
+# Upgrading Instruction
+
+## Overview
+Karmada uses the [semver versioning](https://semver.org/) and each version in the format of v`MAJOR`.`MINOR`.`PATCH`:
+- The `PATCH` release does not introduce breaking changes.
+- The `MINOR` release might introduce minor breaking changes with a workaround.
+- The `Major` release might introduce backward-incompatible behavior changes.
+
+## Regular Upgrading Process
+### Upgrading APIs
+For releases that introduce API changes, the Karmada API(CRD) that Karmada components rely on must upgrade to keep consistent.
+
+Karmada CRD is composed of two parts:
+- bases: The CRD definition generated via API structs.
+- patches: conversion settings for the CRD.
+
+In order to support multiple versions of custom resources, the `patches` should be injected into `bases`.
+To achieve this we introduced a `kustomization.yaml` configuration then use `kubectl kustomize` to build the final CRD.
+
+The `bases`,`patches`and `kustomization.yaml` now located at `charts/_crds` directory of the repo.
+
+#### Manual Upgrade API
+
+**Step 1: Get the Webhook CA certificate**
+The CA certificate will be injected into `patches` before building the final CRD.
+We can retrieve it from the `MutatingWebhookConfiguration` or `ValidatingWebhookConfiguration` configurations, e.g:
+```bash
+kubectl get mutatingwebhookconfigurations.admissionregistration.k8s.io mutating-config
+```
+Copy the `ca_string` from the yaml path `webhooks.name[x].clientConfig.caBundle`, then replace the `{{caBundle}}` from
+the yaml files in `patches`. e.g:
+```bash
+sed -i'' -e "s/{{caBundle}}/${ca_string}/g" ./"charts/_crds/patches/webhook_in_resourcebindings.yaml"
+sed -i'' -e "s/{{caBundle}}/${ca_string}/g" ./"charts/_crds/patches/webhook_in_clusterresourcebindings.yaml"
+```
+
+**Step2: Build final CRD**
+Generate the final CRD by `kubectl kustomize` command, e.g:
+```bash
+kubectl kustomize ./charts/_crds 
+```
+Or, you can apply to `karmada-apiserver` by:
+```bash
+kubectl kustomize ./charts/_crds | kubectl apply -f -
+```
+
+### Upgrading Components
+Components upgrading is composed of image version update and possible command args changes.
+
+> For the argument changes please refer to `Details Upgrading Instruction` below.
+
+## Details Upgrading Instruction
+### [v0.8 to v0.9](./v0.8-v0.9.md)

docs/upgrading/v0.8-v0.9.md

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+# v0.8 to v0.9
+
+Nothing special other than the [Regular Upgrading Process](./README.md).
+
+## Upgrading Notable Changes
+Please refer to [v0.9.0 Release Notes](https://github.com/karmada-io/karmada/releases/tag/v0.9.0).