docs(guide): Add a quickstart section

guide/src/SUMMARY.md

@@ -1,3 +1,3 @@
 # Summary
 
-- [Setup](setup.md)
+- [Quickstart](quickstart.md)

guide/src/quickstart.md

@@ -0,0 +1,78 @@
+# Quickstart
+
+Here is a basic example of using Heron with 2d graphics. It draws a green box
+that falls down due to gravity:
+
+```rust,no_run
+use bevy::prelude::*;
+use heron::*;
+
+#[bevy_main]
+fn main() {
+    App::build()
+        .add_plugins(DefaultPlugins)
+        .add_plugin(PhysicsPlugin::default()) // Add the Heron plugin
+        .add_resource(Gravity::from(Vec3::new(0.0, -300.0, 0.0))) // Define gravity
+        .add_startup_system(spawn.system())
+        .run();
+}
+
+fn spawn(commands: &mut Commands, mut materials: ResMut<Assets<ColorMaterial>>) {
+    // Ensure we can see things
+    commands.spawn(Camera2dBundle::default());
+
+    // the size of our sprite
+    let size = Vec2::new(30.0, 30.0);
+    commands
+        //  here we add a Sprite. We can add any bundle of our choice; the
+        // only required component is a GlobalTransform
+        .spawn(SpriteBundle {
+            sprite: Sprite::new(size),
+            material: materials.add(Color::GREEN.into()),
+            transform: Transform::from_translation(Vec3::new(0.0, 200.0, 0.0)),
+            ..Default::default()
+        })
+        // Make it a physics body, by attaching a collision shape
+        .with(Body::Cuboid {
+            // let the size be consistent with our sprite
+            half_extends: size.extend(0.0) / 2.0,
+        });
+}
+```
+
+If you create a new project using `cargo init` you can replace `main.rs`
+with this code. You should also add this to your projects's `Cargo.toml`:
+
+```toml
+[dependencies]
+bevy = { version = "0.4"} 
+heron = { version = "0.2.0", default-features = false, features = ["2d"] }
+```
+
+Heron defaults to 3d. To make sure we run it in 2d mode we have to configure it
+in `Cargo.toml`; we need to turn off the default features and enable `2d`.
+
+If you then run `cargo run` you should see the green box fall.
+
+## Explanation
+
+We create a normal `Bevy` app. To enable Heron we must add the `PhysicsPlugin`.
+Optionally you can add a `Gravity` resource, as we do here.
+
+We can spawn our physics entities in the startup system as we do here.
+
+We also need to set up the camera bundle to 2d so that we can see something.
+
+We then spawn a `SpriteBundle` in the normal Bevy style; here we generate a
+very basic green `box` sprite and place it on `x` and `y` coordinates. Note
+that we have to use `Vec3` even though we are in 2d space. In 2d we simply set
+the `z` coordinate to zero always.
+
+To make it work with the physics engine, we must add a Heron `Body` component.
+In this case we add a cuboid (rectangular in 2d) collision shape.
+
+And that's all there is to it! Heron, using the Rapier physics engine, makes
+your sprite behave according to physics!
+
+You can do a lot more with Heron but this should get you started!
+