Add initial documentation for --vm-driver=none

Author: tstromberg

docs/vmdriver-none.md

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+# vm-driver=none
+
+## Overview
+
+This document is written for system integrators who are familiar with minikube, and wish to run it within a customized VM environment.
+
+`--vm-driver=none` allows advanced minikube users to skip VM creation, allowing minikube to be run on an user-supplied VM.
+
+## What operating systems are supported?
+
+`--vm-driver=none` supports releases of Debian, Fedora, and buildroot that are less than 2 years old. 
+
+While the standard minikube guest VM uses buildroot, minikube integration tests are also regularly run against Debian 9 for compatibility. In practice, any systemd-based modern distribution is likely to work, and we will happily accept pull requests which improve compatibility with other systems.
+
+## Should vm-driver=none be used on a personal development machine? No.
+
+No. Please do not do this, ever.
+
+minikube was designed to run Kubernetes within a dedicated VM, and when used with `--vm-driver=none`, may overwrite system binaries, configuration files, and system logs. Executing `minikube --vm-driver=none` outside of a VM could result in data loss, system instability and decreased security.
+
+Usage of `--vm-driver=none` outside of a VM could also result in services being exposed in a way that may make them accessible to the public internet. Even if your host is protected by a firewall, these services still be vulnerable to [CSRF](https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)) or [DNS rebinding](https://en.wikipedia.org/wiki/DNS_rebinding) attacks.
+
+## Can vm-driver=none be used outside of a VM?
+
+Yes, but only after appropriate security and reliability precautions have been made. `minikube --vm-driver=none` assumes  complete control over the environment is is executing within, and may overwrite system binaries, configuration files, and system logs. 
+
+The host running `minikube --vm-driver=none` should be:
+
+* Isolated from the rest of the network with a firewall
+* Disposable and easily reprovisioned, as this mode may overwrite system binaries, configuration files, and system logs
+
+If you find yourself running a web browser on the same host running `--vm-driver=none`, please see __Should vm-driver=none be used on a personal development machine? No.__
+
+## Known Issues
+
+* You cannot run more than one `--vm-driver=none` instance on a single host #2781
+* `--vm-driver=none` deletes other local docker images #2705
+* `--vm-driver=none` fails on distro's which do not use systemd #2704
+* Many `minikube` commands are not supported, such as: `dashboard`, `mount`, `ssh`, `stop` #3127