added note about Vagrant on Linux
added note about Vagrant on Linux

heartsucker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,ageis/securedrop,garrettr/securedrop,garrettr/securedrop,micahflee/securedrop,ageis/securedrop,conorsch/securedrop,garrettr/securedrop,ehartsuyker/securedrop,conorsch/securedrop,micahflee/securedrop,micahflee/securedrop,heartsucker/securedrop,conorsch/securedrop,heartsucker/securedrop,heartsucker/securedrop,ageis/securedrop,conorsch/securedrop,micahflee/securedrop,ageis/securedrop,ehartsuyker/securedrop