doc: man-page for size command

Provide a man-page for the size command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 528b3c7..6f45fd3 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -38,6 +38,7 @@
    pstore
    qfw
    sbi
+   size
    true
    scp03
    reset