Add .nobackup info.

index.md

@@ -105,6 +105,9 @@ meta-raspberrypi layer as well as its dependencies are needed. Start
 by creating the following directory structure to contain the code:
 
     mkdir -p ~/yocto/sources
+    # Yocto creates a whole lot of files. At least on Lysators
+    # systems it's a good idea to exclude them from the backup.
+    touch ~/yocto/sources/.nobackup
 
 After the command above has been run, there should be a folder in the
 home directory named Yocto, with one subfolder named sources. The