infra/common: document tap interface setup procedure

The platform FVP supports virtual ethernet interface to allow networking
support to be usable for the software executed by the FVP. If support
for networking is required, the host TAP interface has to be setup
before the FVP is launched. Document the steps required to setup the
TAP interface on this host machine.

Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
Change-Id: I7599711781fa2ca1c18ff3bdc70aaf67614d605a