doc: man-page base command

Provide a description of the base command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index fbb2c04..57bf4fe 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -11,6 +11,7 @@
 .. toctree::
    :maxdepth: 1
 
+   base
    bootefi
    bootmenu
    button