doc: Add mention of the /config binding

The devicetree binding files are in their own directory and use a simple
text format. Add a link for the binding for the /config node, since it
is otherwise hard to find.

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/develop/config_binding.rst b/doc/develop/config_binding.rst
new file mode 100644
index 0000000..c90e99c
--- /dev/null
+++ b/doc/develop/config_binding.rst
@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+U-Boot configuration node
+=========================
+
+U-Boot supports a number of runtime configuration options which can be selected
+in the devicetree.
+
+These are documented in
+:download:`The /config node <../../doc/device-tree-bindings/config.txt>`.
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 2a32645..24056b1 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -12,6 +12,7 @@
    bloblist
    ci_testing
    commands
+   config_binding
    devicetree/index
    driver-model/index
    global_data