I have moved the post to the correct forum as it is not related to the layout.
There is no specific way, it is just information to appear below a table. I would just set up a style that differs from the normal text and is close to the table. Perhaps even merge all the cells in the bottom row and enter it there.
In addition to what my colleague said, I might also suggest that you investigate using a popup for the information to be presented in the footnote. Depending on the length of the table, it could be more helpful to your end user as they remain looking at the information, click the link and see the footnote.