README.md: remove unsupported releases
Also add a link to Kubernetes versioning which is the one we follow.
## Compatibility matrix: CRI-O <-> Kubernetes clusters
+CRI-O and Kubernetes follow the same release cycle and deprecation policy. For more information visit the [Kubernetes versioning documentation](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/release/versioning.md).
+
| Version - Branch | Kubernetes branch/version | Maintenance status |
|------------------------------|---------------------------------|--------------------|
-| CRI-O 1.0.x - release-1.0 | Kubernetes 1.7 branch, v1.7.x | = |
-| CRI-O 1.8.x - release-1.8 | Kubernetes 1.8 branch, v1.8.x | = |
-| CRI-O 1.9.x - release-1.9 | Kubernetes 1.9 branch, v1.9.x | = |
| CRI-O 1.10.x - release-1.10 | Kubernetes 1.10 branch, v1.10.x | = |
| CRI-O 1.11.x - release-1.11 | Kubernetes 1.11 branch, v1.11.x | = |
| CRI-O 1.12.x - release-1.12 | Kubernetes 1.12 branch, v1.12.x | = |
