Add example of a replica assignment file for fluvio topic create

[tjtelan]

Noticed this option in fluvio topic create

$ fluvio topic create -h
[...]
    -f, --replica-assignment <file.json>    Replica assignment file

I've never used this option before, and I don't know what the format should look like. No one else will be patient enough to figure this out, so we should show this off with an example.