[docs] Add comment explaining required deps

[beyarkay]

This is a no-code commit. The pagination_async example doesn't explicitly say that the futures and futures_util crates are required, so I had to do a lil' hunting to figure out what crates I was missing when I tried to copy-paste the code.

This commit just adds a comment explaining how to add the required dependencies.

[ramsayleung]

Thanks for your contribution :)