Add documentation on using `composer create-project` to start a new project

[NielsdeBlaauw]

I've currently used this sequence to base a new project on clarkson-theme:

mkdir theme_name
cd theme_name
composer create-project level-level/clarkson-theme --repository '{ "type": "git", "url": "https://github.com/level-level/Clarkson-Theme" }' --stability dev ./
git init
git remote add origin [[NEW_REPO_URL]]
git add -A
git commit -a -m "New project based on clarkson-theme"
git push -u origin master