Update readme for version compatibility with Kubernetes 1.12+

Author: davidz627

README.md

@@ -1,6 +1,8 @@
 WARNING: This driver is in ALPHA currently. This means that there may be
 potentially backwards compatability breaking changes moving forward. Do NOT use
-this drive in a production environment in its current state.
+this driver in a production environment in its current state.
+
+WARNING: The ALPHA driver is NOT compatible with Kubernetes versions 1.12+.
 
 DISCLAIMER: This is not an officially supported Google product
 
@@ -23,7 +25,10 @@ Latest image: `gcr.io/google-containers/volume-csi/gcp-compute-persistent-disk-c
 This plugin is compatible with CSI versions [v0.2.0](https://github.com/container-storage-interface/spec/blob/v0.2.0/spec.md) and [v0.3.0](https://github.com/container-storage-interface/spec/blob/v0.3.0/spec.md)
 
 ### Kubernetes Compatibility
-This plugin can be used as-is beginning with Kubernetes v1.10.5
+| GCE PD CSI Driver\Kubernetes Version | 1.10.5 - 1.11 | 1.12+ |
+|--------------------------------------|---------------|------|
+| v0.1.0.alpha (stable)                | yes           | no   |
+| dev                                  | no            | yes  |
 
 ### Known Issues
 See Github [Issues](https://github.com/kubernetes-sigs/gcp-compute-persistent-disk-csi-driver/issues)