readme: clarify options, esp. regarding security.
readme: clarify options, esp. regarding security.

See also: expressjs/csurf#12

expressjs/session,cgcardona/session,victorenator/session,aboutdotme/session,globesherpa/session,wesleytodd/session,jasonkarns/session,digitalartcrew/session,mig1098/session,soulman-is-good/session,tianyk/express-session-review,saighost/session,happysj/session,hl198181/session,coolaj86/session,danielgindi/session,MadisonReed/session,tianyk/express-session-research,gabeio/session,Jpunt/session,sielay/session,cnwhy/session,socialrank/express-session,fracmak/session