Document what's missing in containerd-for-processes

[gcapizzi]

While our containerd-for-processes mode is good enough to run Cloud Foundry, it doesn't implement the full Garden API yet. Let's make sure this is clearly documented and list the features that are still missing.

We might also want to revisit this sentence:

At time of writing, we have confidence that containerd-for-processes mode is production ready

by specifying that we mean "in Cloud Foundry".

[MarcPaquette]

Hi @gcapizzi,
What are the next steps on this issue?

Thanks,
Marc