How to add a footnote to a table:

Using the table in their sample file, I added a footnote after \end{center}.

\begin{tabular}{lccc}\hline
$a/d$ & $M=4$ & $M=8$ & Callan \etal \\\hline%\\%[3pt]
0.1~~ & 1.56905 & ~~~1.56$^a$~ & 1.56904\\
0.3~~ & 1.50484 & ~~1.504 & 1.50484\\
0.55~ & 1.39128 & ~~1.391 & 1.39131\\
0.7~~ & 1.32281 & ~10.322 & 1.32288\\
0.913 & 1.34479 & 100.351 & 1.35185\\\hline
\end{tabular}
\end{center}
$^a$This is a footnote.
\end{table}