To include a non composer repository you need to set up a package repository. Which would give you something like:
{
"repositories": [
{
"type": "package",
"package": {
"name": "fguillot/picoFeed",
"version": "dev-master",
"source": {
"url": "https://github.com/fguillot/picoFeed",
"type": "git",
"reference": "origin/master"
}
}
}
],
"require": {
"fguillot/picoFeed": "dev-master"
}
}