Update README to reflect Read the Docs migration
Update README to reflect Read the Docs migration

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