Add documentation for @inject decorator

[e2tox]

@Inject will do dependency injection when user create a new agent class.

e.g

class Calculator {
   add(left:number, right: number): number {
     return left + right;
   }
}

@agent('awesome')
class MyAgent {

   @inject(Calculator)
   calculator: Calculator

}


const agent = new MyAgent();

console.log(agent.calculator.add(1,3));
// 4

[e2tox]

This @Inject decorator already included in the latest release on npm