Add README and MIT license - #1

Adds a README and an MIT license for the behaviour tree package. No runtime or editor code is changed.

package.json has no license field, so the manifest is unchanged.

The README is written from the current sources:

• One-line summary of com.deathbygravitystudio.behaviour.tree: a ScriptableObject behaviour tree ticked by Behaviours.BT.Behaviour, edited in Window > Behaviour Tree Window.
• Features that exist in code: tree assets, task states, built-in composites, leaves, conditions, services, custom tasks with RuntimeData / TaskColor / BTVar, and the editor window.
• Unity version: the manifest does not declare one. The editor uses C# 9, which Unity compiles from 2021.2.
• Installation from https://github.com/denchi/UnityBehaviorTree.git, plus the required sibling com.deathbygravitystudio.behaviour.vars (https://github.com/denchi/UnityBehaviorVars.git) at 0.1.0.
• A quick-start custom Task and a Behaviour driver using setBool and restart.
• How this package relates to UnityBehaviorHFSM, UnityBehaviorGraph, UnityBehaviorVars, and UnityBehaviorDialogs: they share Vars, and only Vars is a dependency of this package.
• Licence section pointing at LICENSE (Copyright 2026 Denis Chiriac / Death By Gravity Studio).