Add second example to QuickStart guide.

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

doc/examples/QuickStart_example.cpp

@@ -2,12 +2,14 @@
 #include <Eigen/Dense>
 
 using Eigen::MatrixXd;
+using Eigen::VectorXd;
 
 int main(int, char *[])
 {
-  MatrixXd m(2,3);
-  m(0,0) = -3;
-  m(1,0) = 1.5;
-  m.rightCols(2) = MatrixXd::Identity(2,2);
+  MatrixXd m(2,2);
+  m(0,0) = 3;
+  m(1,0) = 2.5;
+  m(0,1) = -1;
+  m(1,1) = m(1,0) + m(0,1);
   std::cout << 2*m << std::endl;
 }