Add an example to draw a rectangle (#2957)

# Objective

Every time I come back to Bevy I face the same issue: how do I draw a rectangle again? How did that work? So I go to https://github.com/bevyengine/bevy/tree/main/examples in the hope of finding literally the simplest possible example that draws something on the screen without any dependency such as an image. I don't want to have to add some image first, I just quickly want to get something on the screen with `main.rs` alone so that I can continue building on from that point on. Such an example is particularly helpful for a quick start for smaller projects that don't even need any assets such as images (this is my case currently).

Currently every single example of https://github.com/bevyengine/bevy/tree/main/examples#2d-rendering (which is the first section after hello world that beginners will look for for very minimalistic and quick examples) depends on at least an asset or is too complex. This PR solves this.
It also serves as a great comparison for a beginner to realize what Bevy is really like and how different it is from what they may expect Bevy to be. For example for someone coming from [LÖVE](https://love2d.org/), they will have something like this in their head when they think of drawing a rectangle:
```lua
function love.draw()
    love.graphics.setColor(0.25, 0.25, 0.75);
    love.graphics.rectangle("fill", 0, 0, 50, 50);
end
```
This, of course, differs quite a lot from what you do in Bevy. I imagine there will be people that just want to see something as simple as this in comparison to have a better understanding for the amount of differences.

## Solution

Add a dead simple example drawing a blue 50x50 rectangle in the center with no more and no less than needed.

Cargo.toml

@@ -114,6 +114,10 @@ path = "examples/2d/contributors.rs"
 name = "many_sprites"
 path = "examples/2d/many_sprites.rs"
 
+[[example]]
+name = "rect"
+path = "examples/2d/rect.rs"
+
 [[example]]
 name = "sprite"
 path = "examples/2d/sprite.rs"

examples/2d/rect.rs

@@ -0,0 +1,20 @@
+use bevy::prelude::*;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .run();
+}
+
+fn setup(mut commands: Commands) {
+    commands.spawn_bundle(OrthographicCameraBundle::new_2d());
+    commands.spawn_bundle(SpriteBundle {
+        sprite: Sprite {
+            color: Color::rgb(0.25, 0.25, 0.75),
+            custom_size: Some(Vec2::new(50.0, 50.0)),
+            ..Default::default()
+        },
+        ..Default::default()
+    });
+}

examples/README.md

@@ -84,6 +84,7 @@ Example | File | Description
 --- | --- | ---
 `contributors` | [`2d/contributors.rs`](./2d/contributors.rs) | Displays each contributor as a bouncy bevy-ball!
 `many_sprites` | [`2d/many_sprites.rs`](./2d/many_sprites.rs) | Displays many sprites in a grid arragement! Used for performance testing.
+`rect` | [`2d/rect.rs`](./2d/rect.rs) | Renders a rectangle
 `sprite` | [`2d/sprite.rs`](./2d/sprite.rs) | Renders a sprite
 `sprite_sheet` | [`2d/sprite_sheet.rs`](./2d/sprite_sheet.rs) | Renders an animated sprite
 `text2d` | [`2d/text2d.rs`](./2d/text2d.rs) | Generates text in 2d