I don’t use the standart raspoppy but I guess (@Theo to confirm) :
Find the snap directory and …/snap/examples
You should put your .xml and add a line in the EXAMPLES files.
The management of Snap projects file need a lot of improvments (help is welcome)…
Basically, you have to copy them (or with a symlink) in the /home/poppy/dev/puppet-master/snap/Examples/ folder, and add there name in /home/poppy/dev/puppet-master/snap/Examples/EXAMPLES file. If you want to add again standard Snap project examples, you just have to add them in the EXAMPLE file.