No instructions on running VM on Linux host

[jonahgraham]

At the moment the readme has Windows and Mac instructions, but not Linux.

[jonahgraham]

Until this gets converted into step-by-steps, here are some notes:

• On Ubuntu 18.04 the virtual box that is included by default is 5.x, download and install 6.x from virtualbox instead
• There does not seem to be host networks by default in virtualbox on Linux, so File -> Host Network Manager, then Network -> Create. In the newly created network enable the DHCP server
• the VM may start in error related to host network, I have resolved this by simply opening the Machine's network settings and pressing ok (with perhaps a dummy edit).
• In addition to installing extension pack for USB (As you need to do on other platforms) you also have to add current user to the vboxusers group (e.g. on Ubuntu: sudo usermod -aG vboxusers $USER and then log-off and on again for new groups to take effect)

[alfredoh1234]

added to instructions on repo. Thanks @jonahgraham !