Changing examples to use makefile

- Testing a modified version of the makefile from raylib Instead of multiple project files for examples.
- Fixed readme example

Examples/models/models_billboard.cs

@@ -0,0 +1,7 @@
+using Raylib;
+using static Raylib.Raylib;
+
+public partial class Examples
+{
+    /*******************************************************************************************
    *
    *   raylib [models] example - Drawing billboards
    *
    *   This example has been created using raylib 1.3 (www.raylib.com)
    *   raylib is licensed under an unmodified zlib/libpng license (View raylib.h for details)
    *
    *   Copyright (c) 2015 Ramon Santamaria (@raysan5)
    *
    ********************************************************************************************/
    
    
    public static void Main()
    {
        // Initialization
        //--------------------------------------------------------------------------------------
        int screenWidth = 800;
        int screenHeight = 450;
    
        InitWindow(screenWidth, screenHeight, "raylib [models] example - drawing billboards");
    
        // Define the camera to look into our 3d world
        Camera camera = { 0 };
        camera.position = new Vector3( 5.0f, 4.0f, 5.0f );;
        camera.target = new Vector3( 0.0f, 2.0f, 0.0f );;
        camera.up = new Vector3( 0.0f, 1.0f, 0.0f );;
        camera.fovy = 45.0f;
        camera.type = CAMERA_PERSPECTIVE;
        
    
        Texture2D bill = LoadTexture("resources/billboard.png");     // Our texture billboard
        Vector3 billPosition = { 0.0f, 2.0f, 0.0f };                 // Position where draw billboard
        
        SetCameraMode(camera, CAMERA_ORBITAL);  // Set an orbital camera mode
    
        SetTargetFPS(60);                       // Set our game to run at 60 frames-per-second
        //--------------------------------------------------------------------------------------
    
        // Main game loop
        while (!WindowShouldClose())            // Detect window close button or ESC key
        {
            // Update
            //----------------------------------------------------------------------------------
            UpdateCamera(&camera);              // Update camera
            //----------------------------------------------------------------------------------
    
            // Draw
            //----------------------------------------------------------------------------------
            BeginDrawing();
    
                ClearBackground(RAYWHITE);
    
                BeginMode3D(camera);
                
                    DrawBillboard(camera, bill, billPosition, 2.0f, WHITE);
                    
                    DrawGrid(10, 1.0f);        // Draw a grid
    
                EndMode3D();
    
                DrawFPS(10, 10);
    
            EndDrawing();
            //----------------------------------------------------------------------------------
        }
    
        // De-Initialization
        //--------------------------------------------------------------------------------------
        UnloadTexture(bill);        // Unload texture
    
        CloseWindow();              // Close window and OpenGL context
        //--------------------------------------------------------------------------------------
    
        return 0;
    }    
+}