Add explanation of what \Gamma is in eq.

Rev number 1 reported that \Gamma wasn't introduce after its first
introduction in page 2.
We added a sentence that says that \Gamma is the boundary

tex/methods.tex

@@ -47,7 +47,8 @@ \subsection{Scattering of small particles} \label{sec:scattering_small}
 \end{align}
 %
 Equation \eqref{eq:electrostatic_scatter} is an electrostatic equation 
-with an imposed electric field $\mathbf{E}_i=-\nabla\phi_i$.
+with an imposed electric field $\mathbf{E}_i=-\nabla\phi_i$, where $\Gamma$ is
+is the boundary between regions $\Omega_1$ and $\Omega_2$.
 
 \begin{figure}%[h] %  figure placement: here, top, bottom, or page
    \centering