Depends on what you mean by a "footnote".

Insert|Comment will let you associate a comment with the cell. This
comment will be displayed when your mouse cursor points at the red
triangle that will appear at the upper left corner of the cell.
Tools|Options|View|Comment & Indicator
will leave the comment visible so that it will appear in print-outs (you
can resize and drag to control what portions of the worksheet are
covered by the comment box)

You can write descriptive comments in any other cell to describe the
calculation.

If you want a footnote ID symbol in the cell with the formula, to tie
the cell to the descriptive comments in another cell, then things get a
bit dicey. Formatting cannot be applied to only a portion of the output
of a formula. You could use the formula
=SUM()&CHAR(134)
but the result will be text (can't be used by other formulas) instead of
a number, and the daggar symbol (CHAR(134)) will be in-line, not a
superscript. You can use the TEXT function to control formatting of the
SUM result.

You could get a similar result yet keep the cell result numeric by
appending the character in a custom format, such as
General"a"
You could get the daggar symbol by typing Alt+0134 instead of the letter
"a" between the double quotes in the custom format.

Jerry