add version matrix (#716)

Author: tg123

README.md

@@ -147,6 +147,24 @@ cd ${REPO_DIR}/csharp/src/KubernetesClient
 ${GEN_DIR}/openapi/csharp.sh generated ../csharp.settings
 ```
 
+# Version Compatibility 
+
+| SDK Version | Kubernetes Version | .NET Targeting                        |
+|-------------|--------------------|---------------------------------------|
+| 6.0         | 1.22               | netstandard2.1;net5                   |
+| 5.0         | 1.21               | netstandard2.1;net5                   |
+| 4.0         | 1.20               | netstandard2.0;netstandard2.1         |
+| 3.0         | 1.19               | netstandard2.0;net452                 |
+| 2.0         | 1.18               | netstandard2.0;net453                 |
+| 1.6         | 1.16               | netstandard1.4;netstandard2.0;net452; |
+| 1.4         | 1.13               | netstandard1.4;net451                 |
+| 1.3         | 1.12               | netstandard1.4;net452                 |
+
+ * Starting form `2.0`, [dotnet sdk versioning](https://github.com/kubernetes-client/csharp/issues/400) adopted
+ * `Kubernetes Version` here means the version sdk models and apis were generated from
+ * Kubernetes api server guarantees the compatibility with `n-2` version. for exmaple, 1.19 based sdk should work with 1.21 cluster, but no guarantee works with 1.22 cluster. see also <https://kubernetes.io/releases/version-skew-policy/>
+
+
 ## Contributing
 
 Please see [CONTRIBUTING.md](CONTRIBUTING.md) for instructions on how to contribute.