Update README for 19.15.0a1 client

Author: roycaihw

README.md

@@ -88,6 +88,7 @@ supported versions of Kubernetes clusters.
 - [client 12.y.z](https://pypi.org/project/kubernetes/12.0.1/): Kubernetes 1.15 or below (+-), Kubernetes 1.16 (✓), Kubernetes 1.17 or above (+-)
 - [client 17.y.z](https://pypi.org/project/kubernetes/17.17.0/): Kubernetes 1.16 or below (+-), Kubernetes 1.17 (✓), Kubernetes 1.18 or above (+-)
 - [client 18.y.z](https://pypi.org/project/kubernetes/18.20.0/): Kubernetes 1.17 or below (+-), Kubernetes 1.18 (✓), Kubernetes 1.19 or above (+-)
+- [client 19.y.z](https://pypi.org/project/kubernetes/19.15.0a1/): Kubernetes 1.18 or below (+-), Kubernetes 1.19 (✓), Kubernetes 1.20 or above (+-)
 
 > See [here](#homogenizing-the-kubernetes-python-client-versions) for an explaination of why there is no v13-v16 release.
 
@@ -128,6 +129,7 @@ between client-python versions.
 | 17.0            | Kubernetes main repo, 1.17 branch    | ✓                             |
 | 18.0 Alpha/Beta | Kubernetes main repo, 1.18 branch    | ✗                             |
 | 18.0            | Kubernetes main repo, 1.18 branch    | ✓                             |
+| 19.0 Alpha/Beta | Kubernetes main repo, 1.19 branch    | ✓                             |
 
 > See [here](#homogenizing-the-kubernetes-python-client-versions) for an explaination of why there is no v13-v16 release.