DifferentialEquationsLecture2

Add lecture notes from the second lecture on differential equations.

src/differential_equations_2.md

@@ -350,6 +350,8 @@ Notice that when stating the ordinary differential equation, it is specified
 along with it's boundary conditions. Note that in contrast to an initial value
 problem, a boundary value problem does not always have a solution. 
 
+<img src="figures/DE2_1.png" width="700">
+