Document the inability to use `XmlAttributeAttribute` with Noda Time XML serialization.
Document the inability to use `XmlAttributeAttribute` with Noda Time
XML serialization.

This fixes issue 259.

zaccharles/nodatime,nodatime/nodatime,malcolmr/nodatime,nodatime/nodatime,BenJenkinson/nodatime,zaccharles/nodatime,zaccharles/nodatime,zaccharles/nodatime,zaccharles/nodatime,malcolmr/nodatime,jskeet/nodatime,malcolmr/nodatime,zaccharles/nodatime,BenJenkinson/nodatime,malcolmr/nodatime,jskeet/nodatime