upl: Add initial documentation

Add some documentation to explain the basic concept along with a link
to the full spec.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 43d430c..215ffd0 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1716,6 +1716,7 @@
 F:	boot/upl*
 F:	cmd/upl.c
 F:	common/spl/spl_upl.c
+F:	doc/usage/upl.rst
 F:	doc/usage/cmd/upl.rst
 F:	include/upl.h
 F:	test/boot/upl.c