updated developer docs w.r.t. pre-commit hook
updated developer docs w.r.t. pre-commit hook

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