Warn users: Fusion = no go

[jahio]

I was (mistakenly) under the impression that, "oh hey, no big deal I can just vagrant up this thing with Fusion as the provider!" Yeeeah...I learned that was a Bad Idea(TM):

Bringing machine 'default' up with 'vmware_fusion' provider...
==> default: Box 'cloudfoundry/no-support-for-bosh-lite-on-fusion' could not be found. Attempting to find and install...
    default: Box Provider: vmware_desktop, vmware_fusion, vmware_workstation
    default: Box Version: >= 0
The box 'cloudfoundry/no-support-for-bosh-lite-on-fusion' could not be found or
could not be accessed in the remote catalog. If this is a private
box on HashiCorp's Atlas, please verify you're logged in via
`vagrant login`. Also, please double-check the name. The expanded
URL and error message are shown below:

URL: ["https://atlas.hashicorp.com/cloudfoundry/no-support-for-bosh-lite-on-fusion"]
Error: The requested URL returned error: 404 Not Found

This PR updates the readme with a short bolded sentence (to catch the eye of your average casual skimmer, hopefully) explaining that Fusion is unsupported and to use VirtualBox instead.

[cfdreddbot]

Hey jaustinhughey!

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[sykesm]

@cppforlife: what is preventing vmware boxes from getting published? They used be part of the pipeline and got dropped with the move from jenkins to gocd and never restored with the move to concourse.

Thanks.

[cppforlife]

@sykesm mostly CI work. it's not trivial to setup vmware building box. pcfdev team has made some progress of automating it though.