added offer create guide
added offer create guide

ripple/ripple-lib,wilsonianb/ripple-lib,darkdarkdragon/ripple-lib,darkdarkdragon/ripple-lib,ripple/ripple-lib,darkdarkdragon/ripple-lib,ripple/ripple-lib,wilsonianb/ripple-lib,wilsonianb/ripple-lib,ripple/ripple-lib