docs: describe "file" and "local" methods in LIBFOO_SITE_METHOD

Based on description by Thomas Petazzoni at:
http://free-electrons.com/blog/buildroot-2011-11/



Signed-off-by: Nathan Lynch <ntl@pobox.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>