$ git clone git@github.com:atom/atom.git
Cloning into 'atom'...
The authenticity of host 'github.com (192.30.252.129)' can't be established.
RSA key fingerprint is 16:27:ac:a5:76:28:2d:36:63:1b:56:4d:eb:df:a6:48.
Are you sure you want to continue connecting (yes/no)? y
Please type 'yes' or 'no': yes
Warning: Permanently added 'github.com,192.30.252.129' (RSA) to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.
Looks like their README reflects the fact that anyone who could see the repo could also push to it. You should submit a PR :)
EDIT: I am mistaken. I thought cloning git@ meant you could read/write from it (aka allowed to push). Turns out it's just a public key thing. Comment above me is correct.