[INTERNAL] added README file for virtual file system

Just some brain storming the file API of the core must fulfill.

Change-Id: I3e46af3bf58311f5167801c81221b2ede72b2ba2
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1406
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>