Update custom_controls.adoc

src/docs/asciidoc/jme3/advanced/custom_controls.adoc

@@ -33,3 +33,35 @@ To implement game logic for a type of spatial, you will either extend AbstractCo
 
 
 == Usage
+
+Use <<jme3/advanced/custom_controls#,Controls>> to implement the _behaviour of types of game entities_.
+
+*  Use Controls to add a type of behaviour (that is, methods and fields) to individual Spatials. 
+*  Each Control has its own `update()` loop that hooks into `simpleUpdate()`. Use Controls to move blocks of code out of the `simpleUpdate()` loop.
+*  One Spatial can be influenced by several Controls. (Very powerful and modular!) 
+*  Each Spatial needs its own instance of the Control. 
+*  A Control only has access to and control over the Spatial it is attached to.
+*  Controls can be saved as .j3o file together with a Spatial. 
+
+Examples: You can write
+
+*  A WalkerNavControl, SwimmerNavControl, FlyerNavControl… that defines how a type of NPC finds their way around. All NPCs can walk, some can fly, others can swim, and some can all three, etc.
+*  A PlayerNavControl that is steered by user-configurable keyboard and mouse input.
+*  A generic animation control that acts as a common interface that triggers animations (walk, stand, attack, defend) for various entities.
+*  A DefensiveBehaviourControl that remote-controls NPC behaviour in fight situations. 
+*  An IdleBehaviourControl that remote-controls NPC behaviour in neutral situations. 
+*  A DestructionControl that automatically replaces a structure with an appropriate piece of debris after collision with a projectile… 
+
+The possibilities are endless. emoji:smiley
+
+
+== Example Code
+
+Other examples include the built-in RigidBodyControl in JME's physics integration, the built-in TerrainLODControl that updates the terrain's level of detail depending on the viewer's perspective, etc.
+
+Existing examples in the code base include:
+
+*  link:https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-core/src/main/java/com/jme3/animation/AnimControl.java[AnimControl.java] allows manipulation of skeletal animation, including blending and multiple channels.
+*  link:https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-core/src/main/java/com/jme3/scene/control/CameraControl.java[CameraControl.java] allows you to sync the camera position with the position of a given spatial.
+*  link:https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-core/src/main/java/com/jme3/scene/control/BillboardControl.java[BillboardControl.java] displays a flat picture orthogonally, e.g. a speech bubble or informational dialog.
+*  link:https://github.com/jMonkeyEngine/jmonkeyengine/tree/master/jme3-bullet/src/common/java/com/jme3/bullet/control[PhysicsControl] subclasses (such as CharacterControl, RigidBodyControl, VehicleControl) allow you to add physical properties to any spatial. PhysicsControls tie into capabilities provided by the BulletAppState.