chore: add cli readme (#8223)

Signed-off-by: Charles-Edouard Brétéché <charles.edouard@nirmata.com>

cmd/cli/kubectl-kyverno/README.md

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+# kubectl-kyverno
+
+<a href="https://kyverno.io" rel="kyverno.io">![logo](../../../img/Kyverno_Horizontal.png)</a>
+
+This repository contains [Kyverno CLI](https://kyverno.io/docs/kyverno-cli/) source code.
+
+The CLI can be used as a standalone tool or as a kubectl plugin.
+
+## 📙 Documentation
+
+👉 **[Installation](https://kyverno.io/docs/kyverno-cli/#building-and-installing-the-cli)**
+
+👉 **[Installation](https://kyverno.io/docs/kyverno-cli/#cli-commands)**
+
+👉 **[Reference docs](../../../docs/user/cli/kyverno.md)**
+
+## 🙋‍♂️ Help
+
+Use `kyverno --help` to list supported commands and their corresponding flags:
+
+```shell
+To enable experimental commands, KYVERNO_EXPERIMENTAL should be configured with true or 1.
+
+Usage:
+  kyverno [command]
+
+Available Commands:
+  apply       Applies policies on resources.
+  completion  Generate the autocompletion script for the specified shell
+  create      Provides a command-line interface to help with the creation of various Kyverno resources.
+  docs        Generates documentation.
+  help        Help about any command
+  jp          Provides a command-line interface to JMESPath, enhanced with Kyverno specific custom functions.
+  test        Run tests from directory.
+  version     Shows current version of kyverno.
+
+Flags:
+      --add_dir_header                   If true, adds the file directory to the header of the log messages
+      --alsologtostderr                  log to standard error as well as files (no effect when -logtostderr=true)
+  -h, --help                             help for kyverno
+      --log_backtrace_at traceLocation   when logging hits line file:N, emit a stack trace (default :0)
+      --log_dir string                   If non-empty, write log files in this directory (no effect when -logtostderr=true)
+      --log_file string                  If non-empty, use this log file (no effect when -logtostderr=true)
+      --log_file_max_size uint           Defines the maximum size a log file can grow to (no effect when -logtostderr=true). Unit is megabytes. If the value is 0, the maximum file size is unlimited. (default 1800)
+      --logtostderr                      log to standard error instead of files (default true)
+      --one_output                       If true, only write logs to their native severity level (vs also writing to each lower severity level; no effect when -logtostderr=true)
+      --skip_headers                     If true, avoid header prefixes in the log messages
+      --skip_log_headers                 If true, avoid headers when opening log files (no effect when -logtostderr=true)
+      --stderrthreshold severity         logs at or above this threshold go to stderr when writing to files and stderr (no effect when -logtostderr=true or -alsologtostderr=false) (default 2)
+  -v, --v Level                          number for the log level verbosity
+      --vmodule moduleSpec               comma-separated list of pattern=N settings for file-filtered logging
+```
+
+To enable experimental commands, `KYVERNO_EXPERIMENTAL` should be configured with true or 1.
+
+## License
+
+Copyright 2023, the Kyverno project. All rights reserved. Kyverno is licensed under the [Apache License 2.0](LICENSE).
+
+Kyverno is a [Cloud Native Computing Foundation (CNCF) Incubating project](https://www.cncf.io/projects/) and was contributed by [Nirmata](https://nirmata.com/?utm_source=github&utm_medium=repository).