Add exercise3 of coordiante systems chapter to repo.

src/1.getting_started/6.4.coordinate_systems_exercise3/coordinate_systems_exercise3.cpp

@@ -0,0 +1,19 @@
+...
+
+
+glBindVertexArray(VAO);
+for(unsigned int i = 0; i < 10; i++)
+{
+    // calculate the model matrix for each object and pass it to shader before drawing
+    glm::mat4 model = glm::mat4(1.0f);
+    model = glm::translate(model, cubePositions[i]);
+    float angle = 20.0f * i; 
+    if(i % 3 == 0)  // every 3rd iteration (including the first) we set the angle using GLFW's time function.
+        angle = glfwGetTime() * 25.0f;
+    model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f));
+    ourShader.setMat4("model", model);
+    
+    glDrawArrays(GL_TRIANGLES, 0, 36);           
+}
+
+...