Add example for black and white

examples/black_and_white.rs

@@ -0,0 +1,20 @@
+extern crate mini_gl_fb;
+
+use mini_gl_fb::{Config, BufferFormat};
+
+fn main() {
+    let mut fb = mini_gl_fb::get_fancy(Config {
+        window_title: "Hello world!",
+        window_size: (800.0, 600.0),
+        buffer_size: (2, 2),
+        .. Default::default()
+    });
+
+    fb.change_buffer_format::<u8>(BufferFormat::R);
+    fb.use_grayscale_shader();
+
+    let buffer = [128u8, 255, 50, 25];
+    fb.update_buffer(&buffer);
+
+    fb.persist_and_redraw(true);
+}