dm: core: Add documentation for the dm command

Add a description and examples for the dm subcommands.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 8d08ea1..8b98629 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -33,6 +33,7 @@
    cmd/bootz
    cmd/cbsysinfo
    cmd/conitrace
+   cmd/dm
    cmd/echo
    cmd/env
    cmd/event