Installing ungit on Debian Wheezy

Added installation guide for ungit on Debian Wheezy under "Known Issues", 
because Debian's supported packages through their packagemanager are too old.
Are you sure it's a good idea to recommend using `--gitVersionCheckOverride` in the README. The reason that option is necessary in the first place is because only git versions 1.8 and up are officially supported. Wouldn't it be better to recommend upgrading `git` instead of ignoring the warning?