doc: commit the document for regenerating the C client

docs/how-to-regenerate-the-c-client.md

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+# How to regenerate the C client
+
+## Regenerate with [kubernetes-client/gen](https://github.com/kubernetes-client/gen.git):
+
+When a new version of kubernetes is released, you can follow the steps below to regenerate a new version of C client:
+
+Fork and clone the `c` repository:
+
+```bash
+git clone https://github.com/your-name/c.git
+git remote add upstream https://github.com/kubernetes-client/c.git
+```
+
+Clone the `gen` repository:
+
+```bash
+git clone https://github.com/kubernetes-client/gen.git
+```
+
+Prepare the below envronment variables:
+
+```bash
+CLIENT_REPO_ROOT=$HOME/c
+GEN_REPO_ROOT=$HOME/gen
+OUTPUT_DIR=$HOME/generated-kubernetes
+SETTING_FILE=$CLIENT_REPO_ROOT/settings
+NEW_BRANCH=your-branch-name
+```
+
+Create a new branch for the regenerated C client:
+
+```bash
+cd $CLIENT_REPO_ROOT
+git checkout -b $NEW_BRANCH
+```
+
+Update the settings (e.g. version) for the new client in below file if needed:
+
+```
+$CLIENT_REPO_ROOT/settings
+```
+
+Delete the output directory if it already exists:
+```bash
+if [ -e $OUTPUT_DIR -a -d $OUTPUT_DIR ]; then rm -r $OUTPUT_DIR; fi
+```
+
+Execute the generating command:
+
+```bash
+cd $GEN_REPO_ROOT/openapi
+./c.sh $OUTPUT_DIR $SETTING_FILE
+```
+
+Copy the generated files to overwrite the files in the C client:
+
+```bash
+cp -rf $OUTPUT_DIR/* $CLIENT_REPO_ROOT/kubernetes/
+```
+
+Check the new/changed/deleted files:
+
+```bash
+cd $CLIENT_REPO_ROOT
+git status
+```
+
+Build and test.
+
+Commit the regnerate client to your repository:
+```bash
+cd $CLIENT_REPO_ROOT
+git add .
+git commit
+git push origin $NEW_BRANCH
+```
+
+Create a Pull Request to merge your regenerated client to upstream repository.
+
+## Regenerate with [openapi-generator](https://github.com/OpenAPITools/openapi-generator)
+
+If you change the code ( e.g. C-libcurl [template](https://github.com/OpenAPITools/openapi-generator/tree/master/modules/openapi-generator/src/main/resources/C-libcurl) ) in [openapi-generator](https://github.com/OpenAPITools/openapi-generator), you can regenerate the client with the current [swagger.json](https://github.com/kubernetes-client/c/blob/master/kubernetes/swagger.json):
+
+Execute the generating command in the directory of  `OpenAPITools/openapi-generator`
+
+```bash
+mvn -B --no-snapshot-updates clean package -DskipTests=true -Dmaven.javadoc.skip=true -Djacoco.skip=true
+
+java -DdebugModels -DdebugOperations -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i $CLIENT_REPO_ROOT/kubernetes/swagger.json -g c -o $OUTPUT_DIR --skip-validate-spec --type-mappings "int-or-string=IntOrString"
+``` 
+
+Copy the generated files to overwrite the files in the C client:
+
+```bash
+cp -rf $OUTPUT_DIR/* $CLIENT_REPO_ROOT/kubernetes/
+```
+
+And then follow the steps above to continue (build, test, commit and PR).