docs(readme): Add cgroups v2 configuration instructions to README

[hareetd]

Fixes #789

[andrewazores]

I find this text a bit confusing. Podman and Docker are separate container runtimes, although Podman (and all other OCI tooling) is heavily inspired by Docker. I don't think it should be necessary to have Docker installed alongside Podman, and the Note: at the bottom is talking about configuring Podman's runc version but links to Docker documentation?

[hareetd]

When I attempted to build/run cryostatio/cryostat-reports#7 my Docker installation complained after enabling cgroups v2 so I had to update it accordingly. This made me think that Podman and Docker were linked when in reality it's likely because I had both installed on the system. The Note: regarding the Podman runtime is because of this confusion. I'll remove any mention of Docker from my changes, thanks.

[andrewazores]

Sounds good. FWIW I don't have Docker installed at all and haven't in quite some time - probably removed since F30/cgroups v1, if not earlier.

[hareetd]

Makes sense, I only installed it since the OperatorHub.io test suite was refusing to work with Podman.