Add second example to QuickStart guide.

Also, some changes suggested by Keir and Benoit on mailing list.

doc/examples/QuickStart_example2_fixed.cpp

@@ -0,0 +1,15 @@
+#include <iostream>
+#include <Eigen/Dense>
+
+using Eigen::Matrix3d;
+using Eigen::Vector3d;
+
+int main(int, char *[])
+{
+  Matrix3d m;
+  for (int rowIndex = 0; rowIndex < 3; ++rowIndex)
+    for (int columnIndex = 0; columnIndex < 3; ++columnIndex)
+      m(rowIndex, columnIndex) = rowIndex + 0.01 * columnIndex;
+  Vector3d v = Vector3d::Ones();
+  std::cout << m * v << std::endl;
+}