Add upgrade notes for 0.5. Closes #655.
Add upgrade notes for 0.5. Closes #655.

MappingKat/tilemill,mbrukman/tilemill,florianf/tileoven,florianf/tileoven,tizzybec/tilemill,paulovieira/tilemill-clima,fxtentacle/tilemill,fxtentacle/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,paulovieira/tilemill-clima,mbrukman/tilemill,Zhao-Qi/tilemill,tizzybec/tilemill,tilemill-project/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Zhao-Qi/tilemill,tizzybec/tilemill,mbrukman/tilemill,paulovieira/tilemill-clima,tizzybec/tilemill,nyimbi/tilemill,isaacs/tilemill,MappingKat/tilemill,mbrukman/tilemill,MappingKat/tilemill,florianf/tileoven,tizzybec/tilemill,nyimbi/tilemill,Zhao-Qi/tilemill,paulovieira/tilemill-clima,fxtentacle/tilemill,tilemill-project/tilemill,mbrukman/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Zhao-Qi/tilemill,Zhao-Qi/tilemill,isaacs/tilemill,isaacs/tilemill,nyimbi/tilemill,nyimbi/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,fxtentacle/tilemill,MappingKat/tilemill,nyimbi/tilemill,MappingKat/tilemill,fxtentacle/tilemill,tilemill-project/tilemill,paulovieira/tilemill-clima