doc: move directory hierarchy to HTML

Move section 'Directory hierarchy' from file README to the HTML
documentation.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 79d7736..a52ad63 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -24,6 +24,7 @@
 .. toctree::
    :maxdepth: 1
 
+   directories
    bloblist
    bootstd
    ci_testing