added a few more general notes

regression/lecture/regression-chapter.tex

@@ -21,6 +21,8 @@
 \subsection{Notes}
 \begin{itemize}
 \item Fig 8.2 right: this should be a chi-squared distribution with one degree of freedom!
+\item Gradient ascent is used by bacteria and worms (and others) to
+  approach food sources along chemical and temperature gradients (using biased random walks).
 \end{itemize}
 
 \subsection{New chapter: non-linear fits}