INSTALL.md: Update due to Debian package now available

Hi,

Before proposing a patch, I would like to discuss what INSTALL.md should read regarding Debian.

My proposal:

State that the package is available via both unstable and testing. (Yes, it's currently not yet in testing, but will be soon.)
Therefore: Remove the first part regarding installation.
Keep the second part for now, as long we don't ship the Nautilus extension in Debian, and update it with the missing steps regarding cloning the repository.

What do you think?

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