binman: Add a tutorial on resolving test-coverage bugs

Provide a short description of how tests work, why they are so critical
and how to resolve gaps in Binman's test coverage.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index eef6353..5734037 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -916,6 +916,7 @@
 M:	Simon Glass <sjg@chromium.org>
 M:	Alper Nebi Yasak <alpernebiyasak@gmail.com>
 S:	Maintained
+F:	doc/develop/binman_tests.rst
 F:	tools/binman/
 
 BLKMAP