README from master doesn't point to 4.8 nor 4.9 README

[fidencio]

Description

README from the master branch points to 4.7 and 4.6 installation instructions, but doesn't mentioned 4.8 one.

Steps to reproduce the issue:

1. Open a browser
2. Go to https://github.com/openshift/sandboxed-containers-operator
3. Read the README

Describe the results you received:

The first thing you'll see is two warnings saying that "if you're using OCP 4.x please follow ...", for both OpenShift 4.6 and 4.7.

Describe the results you expected:

The same info presented for 4.6, and 4.7. to be also present for OpenShift 4.8 and OpenShift 4.9

Additional information you deem important (e.g. issue happens only occasionally):

Not relevant

Output of oc describe kataconfig <your-kataconfig>:

Not relevant

Additional environment details (platform, options, etc.):

Not relevant