First draft for the 5-minute quick start guide to kick off discussions.

doc/examples/QuickStart_example.cpp

@@ -0,0 +1,13 @@
+#include <iostream>
+#include <Eigen/Dense>
+
+using Eigen::MatrixXd;
+
+int main(int, char *[])
+{
+  MatrixXd m(2,3);
+  m(0,0) = -3;
+  m(1,0) = 1.5;
+  m.rightCols(2) = MatrixXd::Identity(2,2);
+  std::cout << 2*m << std::endl;
+}