Docs should note PermGenSpace

[kenrestivo]

I found I needed this in order to make slamhound work with non-trivial ns'es:

export JVM_OPTS="-Xmx5g -XX:+CMSClassUnloadingEnabled -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=128M"

Works fine now, but others may run into trouble, might be good to have that in the docs to warn them.