doc: fatinfo man-page

Provide a man-page for the fatinfo command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index d330b14..b64cfe4 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -28,6 +28,7 @@
    exception
    exit
    false
+   fatinfo
    for
    load
    loady