dm: core: Add documentation on how to debug driver model

Sometimes devices don't appear and it can be confusing. Add a few notes to
help with this situation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: fix 2 issues in the doc and include the doc in the index.rst]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/driver-model/index.rst b/doc/driver-model/index.rst
index ea32c36..6d55774 100644
--- a/doc/driver-model/index.rst
+++ b/doc/driver-model/index.rst
@@ -6,6 +6,7 @@
 .. toctree::
    :maxdepth: 2
 
+   debugging
    design
    fdt-fixup
    fs_firmware_loader