doc: move test/README to HTML documentation

Move test/README to the 'Develop U-Boot' chapter of the HTML documentation.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index b108df8..ed2e73b 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -29,3 +29,4 @@
    :maxdepth: 1
 
    coccinelle
+   testing