man autopkgtest: add a mention about how to run local cross arch testing
Follow-up of this original MP: https://code.launchpad.net/~hyask/autopkgtest/+git/autopkgtest/+merge/456002
Follow-up of this original MP: https://code.launchpad.net/~hyask/autopkgtest/+git/autopkgtest/+merge/456002