Add Cherry pick script guide

Signed-off-by: Xue Mingdi <[email protected]>

docs/developer/cherry_pick.md

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+---
+title: Cherry Pick 
+sidebar_position: 9 
+---
+
+This page explains how to use shell script `hack/cherry_pick_pull.sh` in kubeedge/kubeedge repo to cherry pick the patch to release branch.
+
+
+# Cherry Pick Settings
+
+Before the cherry pick script is executed, need to make sure below setting is correctly set:
+
+1. Set environment variable `GITHUB_USER`
+
+This variable is used to designated GitHub username or Orginazation name. This is the necessary for Pull Request.
+
+```shell
+# set GITHUB_USER
+export GITHUB_USER = <your-github-user>
+```
+
+2. Install `hub` 
+
+As this script reply on `hub` to execute GitHub operations, developer must make sure it is installed.
+
+```shell
+# examine whether the hub is installed successfully
+which hub
+```
+
+If `hub` is not installed, please install it by referring [hub installation](https://github.com/github/hub)
+
+3. Other environment variables setting
+
+- variable `DRY_RUN` is used to skip `git push` and creating PR steps, This is useful for creating patches to a release branch without making a PR.When DRY_RUN is set the script will leave you in a branch containing the commits you cherry-picked.
+
+- variable `REGENERATE_DOCS` is used to regenerate documentation for the target branch after picking the specified commits, This is useful when picking commits containing changes to API documentation. **Note**: Set variable `REGENERATE_DOCS` may affect the cherry pick performance.
+
+- variables `UPSTREAM_REMOTE` (default: upstream) and `FORK_REMOTE` (default: origin) are used to override the default remote names to what you have locally.
+
+4. Make sure the working repo is clean, there is no uncommitted or untracking files.
+
+```shell
+# check whether the current repo is clean
+git status
+```
+
+# Execution steps
+
+1. execute cherry_pick_pull.sh with parameters, at least include `remote branch` and `pr number` (could be multiple pr numbers)
+
+```shell
+# Usage of cherry_pick_pull.sh 
+./cherry_pick_pull.sh <remote branch> <pr-number> ...
+```
+
+for example:
+
+```shell
+# examples to run cherry_pick_pull.sh
+./cherry_pick_pull.sh upstream/release-3.14 12345
+./cherry_pick_pull.sh upstream/release-3.14 12345 56789
+```
+
+If the number of arguments is less than 2, this script will exit with help information.
+
+2. Remote branch will be **automatically** updated to ensure it is the latest version.
+
+3. Script will generate new local branch and PR for cherry-pick **automatically**
+
+4. Script will start cherry-pick, if it detects **conflicts or unmerged files**, it will remind developer to fix the issues, and only execute until the issues fixed.
+
+5. After cherry-pick executed, it will remind to confirm to git push to remote branch.
+
+6. If developer does not want to keep the created local branches, they could be deleted manually.
+
+```shell
+# delete the created local branch
+git checkout ${STARTING_BRANCH}    		#replace variable with developer's branch
+git branch -D ${NEWLY_CREATED_BRANCH}	#replace variable with newly created branch
+```
+