add footnote

sec_proc.tex

@@ -27,7 +27,12 @@ \subsubsection{Hardware Boundaries}\label{ssec:proc_bounds_hardware}
 
 \item \textbf{Short exposures --}
 Special Programs that use short exposures would be limited to the
-minimum exposure time is 1 second (stretch goal: 0.1 seconds).
+minimum exposure time is 1 second (stretch goal: 0.1 seconds\footnote{A short-exposure survey 
+of the bright stars of M67, described in Chapter 10.4 of the 
+Observing Strategy White Paper \citep{2017arXiv170804058L}, suggests using the stretch goal of 
+0.1 second exposures or, if that is not possible, \textit{"custom pixel masks to accurately perform 
+photometry on stars as much as 6 magnitudes brighter than the saturation level"}. 
+This may require User-Generated Processing.}).
 There is a potential hardware boundary that limits the readout rate to 1 
 every 15 seconds, which would affect the image acquisition rate and 
 increase the overheads on short exposures.