add Gentoo to list of distributions

Gentoo is about:
* choice: you choose how you build your software, what licenses you accept (the package manager will block inappropriate packages automatically), what features you want to disable (e.g. dbus, ever tried to remove dbus from a debian system? On gentoo you can, via useflags.)
* control: you build your distro from the ground (without doing the cumbersome work of figuring out dependencies, fixing compile failures etc... gentoo is NOT LFS), nothing happens automagically, you only get what you asked for, easy removal of unneded packages/features, manual configuration
* security: we don't trust other people to compile something for us with an unkown toolchain and no way to determine what was really compiled (undocumented patches?); gentoo has advanced security technologies implemented such as a hardened toolchain and selinux ( http://www.gentoo.org/proj/en/hardened/ ) and has a security team that does official announcements about vulnerabilities and workarounds