Add an OPTIONS chunk to Documentation/git-sparse-checkout.txt

Hi all,

I'm wondering if we could add an OPTIONS chunk to Documentation/git-sparse-checkout.txt?

Cause when I was skimming through the SYNOPSIS and it says "git sparse-checkout <subcommand> [<options>]" but I didn't quite see the options until I read into the COMMANDS.

Perhaps something like this:

OPTIONS

    --[no-]cone

    --[no-]sparse-index

    --stdin

If this is okay to do then I will try to submit a patch later :)