add example to the documentation

doc/example.cpp

@@ -0,0 +1,28 @@
+#include "../src/All"
+
+using namespace std;
+
+template<typename Scalar, typename Derived>
+void foo(const EiObject<Scalar, Derived>& m)
+{
+	cout << "Here's m:" << endl << m << endl;
+}
+
+template<typename Scalar, typename Derived>
+EiScalarProduct<Derived>
+twice(const EiObject<Scalar, Derived>& m)
+{
+	return static_cast<Scalar>(2) * m;
+}
+
+int main(int, char**)
+{
+	EiMatrix2d m;
+	m(0,0)= 1;
+	m(1,0)= 2;
+	m(0,1)= 3;
+	m(1,1)= 4;
+	foo(m);
+	foo(twice(m));
+	return 0;
+}