adds mention of SECUREDROP_ env vars to develop docs
adds mention of SECUREDROP_ env vars to develop docs

conorsch/securedrop,chadmiller/securedrop,jaseg/securedrop,pwplus/securedrop,heartsucker/securedrop,kelcecil/securedrop,micahflee/securedrop,micahflee/securedrop,chadmiller/securedrop,chadmiller/securedrop,kelcecil/securedrop,pwplus/securedrop,ageis/securedrop,ehartsuyker/securedrop,jrosco/securedrop,conorsch/securedrop,heartsucker/securedrop,kelcecil/securedrop,jrosco/securedrop,pwplus/securedrop,ageis/securedrop,heartsucker/securedrop,jrosco/securedrop,heartsucker/securedrop,chadmiller/securedrop,jrosco/securedrop,pwplus/securedrop,jaseg/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,jaseg/securedrop,garrettr/securedrop,pwplus/securedrop,ehartsuyker/securedrop,chadmiller/securedrop,micahflee/securedrop,garrettr/securedrop,kelcecil/securedrop,heartsucker/securedrop,ageis/securedrop,kelcecil/securedrop,conorsch/securedrop,ehartsuyker/securedrop,jaseg/securedrop,garrettr/securedrop,ageis/securedrop,jrosco/securedrop,kelcecil/securedrop,pwplus/securedrop,micahflee/securedrop,conorsch/securedrop,chadmiller/securedrop,jrosco/securedrop,jaseg/securedrop,ehartsuyker/securedrop,conorsch/securedrop,jaseg/securedrop