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.
> It works fine for me on Debian Wheezy

So you can confirm that at least all of the features of ungit you've tried work on whatever version of git you are using. That's great, but it seems like that's more of an argument for relaxing the version restrictions than for recommending that people ignore the restrictions entirely.

> Only reason that version check thing is there is because I'm not actively testing it on < 1.8

So until ungit starts being tested on git versions lower than 1.8, there's no guarantee that everything will continue working for installations with those git versions when ungit is updated, hence the warning.

> We could change it so that when it's < 1.8 it still starts but shows a warning in the UI

Seems reasonable. That's basically just a more user-friendly version of the `--gitVersionCheckOverride` flag though.