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>

(cherry picked from commit d01a8f54b6bbc524021e5a370b97ea870af5d46d)

Plus typo-fix by @enkore.

(cherry picked from commit dfdf590445beb5ff66a19d599ee51b25472c3724)

docs/faq.rst

@@ -321,6 +321,13 @@ for a potential solution.
 
 Please note that this workaround only helps you for backup, not for restore.
 
+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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