Added pinning instructions to Debian section

The old Debian section instructed the user to add unstable to
/etc/apt/sources.list and then run apt-get install fsharp
fsharp-console. Without explicitly pinning unstable in
/etc/apt/preferences to a priority lower than that of the target
release, this method would upgrade the user's system to Debian unstable.
Merge pull request #107 from sbisaacson/debian-pin-fix

Debian pin fix