Added Q80 about free disk space


git-svn-id: svn://svn.rockbox.org/rockbox/trunk@5110 a1c6a512-1295-4272-9138-f99709370657
diff --git a/docs/FAQ b/docs/FAQ
index f8f1d76..69f51d3 100644
--- a/docs/FAQ
+++ b/docs/FAQ
@@ -774,3 +774,11 @@
      files.
 
      No, you can't make Rockbox use the track number id3 info for this.
+
+Q80: Why does Rockbox say that I only have 18Gb free space on my 20Gb disk?
+A80: Because the disk manufacturers have a different definition of Giga than
+     the rest of the computer world does.
+
+     Read more about it here:
+
+     http://personal-computer-tutor.com/abc3/v30/vic30.htm