doc: man-page for the part command

Provide a man-page for the part command.

Signed-off-by: Enric Balletbo i Serra <eballetbo@redhat.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index bbd40a6..cf3666a 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -61,6 +61,7 @@
    cmd/mbr
    cmd/md
    cmd/mmc
+   cmd/part
    cmd/pause
    cmd/pinmux
    cmd/printenv