I added in some info about the install process. I and from what I have seen looking around other people have had the issue of RDP not being an option even after freerdp is installed. It seems that freerdp must be installed first in order for it do show up in remmina's dropdown menu. I also tried installing freerdp first and then remmina and this also did not work. The only install string that worked for me was the one I have added to the wiki. This is on 3.13.6-1-ARCH. with remmina 1.0.0-11 and freerdp 1.0.2-4.