Added FAQ section about backing up root partition

Using borgbackup to backup the root partition works fine, just
remember to exclude non-essential directories.

Signed-off-by: Fredrik Mikker <fredrik@mikker.se>

docs/faq.rst

@@ -353,6 +353,10 @@ How can I restore huge file(s) over a unstable connection?
 If you can not manage to extract the whole big file in one go, you can extract
 all the part files (see above) and manually concatenate them together.
 
+Can i backup my root partition (/) with borg?
+--------------------------------------------
+Backing up your entire root partition works just fine, but remember to exclude directories that make no sense to backup, such as /dev, /proc, /sys, /tmp and /run.
+
 If it crashes with a UnicodeError, what can I do?
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