Document need to delete volumes when using terrraform

[justinsb]

We still need to delete volumes when using the terraform workaround, or else confusing things happen ('deleted' clusters come back)

Also maybe check if we can just require terraform 0.7 somehow (the problem is that TF fails silently with < 0.7)

[justinsb]

Adding documentation, but this is going to be less important now that we are encouraging use of TF 0.7.