Installation: Document optional dependency on blkid

If using `scripts/beesd`, we need `blkid` which is part of util-linux.
It should be available on every distribution but let's document it
anyway.

Signed-off-by: Kai Krakow <kai@kaishome.de>

README.md

@@ -377,6 +377,8 @@ Dependencies
 
 * markdown
 
+* util-linux version that provides `blkid` command for the helper
+  script `scripts/beesd` to work
 
 Setup
 -----