/usr/share is not user-writable and this will need to be added on a per-distro basis.
Moreover, BigPicture has a few issues that make it problematic at boot. #160, for instance. Not to mention the MANY things that cause it to crash. This may be an option in the future, but it's not really that stable of a choice right now.
There's a community-driven project which provides functionality you want.
https://github.com/thor27/steam-login
ubuntu/debian packages: https://launchpad.net/steam-login
Nothing extracted yet.
At login time in Linux you can choose desktop session.
I want to have steam as one of my choices for log in (usable when connecting to TV).
Issue can be resolved by creating .desktop file in /usr/share/xsessions (on all the distros).
like that:
The only thing you need to do is remove "Exit to steam" menu item in that "logon" mode. Besides that, it works perfectly.