doc: button command

Provide a description of the 'button' command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index d0f5a9f..6c4b5b9 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -12,4 +12,5 @@
    :maxdepth: 1
 
    bootmenu
+   button
    pstore