Add some documentation for using stimulus with typescript (#540)

docs/reference/using_typescript.md

@@ -0,0 +1,75 @@
+# Using Typescript
+
+Stimulus itself is written [TypeScript](https://www.typescriptlang.org/)
+and provides types directly over its package.
+The following documentation should make it clear how to define types for
+stimulus properties.
+
+## Define Controller Element Type
+
+By default, the `element` of the controller is from type `Element`.  
+If the element expected type is as example `HTMLFormElement` it can
+be defined this way:
+
+```ts
+import { Controller } from '@hotwired/stimulus';
+
+export default class MyController extends Controller<HTMLFormElement> {
+    submit() {
+        new FormData(this.element)
+    }
+}
+```
+
+## Define Controller Value Type
+
+To define the type of configured values is possible
+over the declare feature of TypeScript:
+
+```ts
+import { Controller } from '@hotwired/stimulus';
+
+export default class MyController extends Controller {
+    static values = {
+        code: String,
+    };
+
+    declare readonly codeValue: string;
+}
+```
+
+> The `declare` avoids override of the exist stimulus property and just define the type for TypeScript.
+
+## Define Controller Target Type
+
+To define the type of configured targets is possible
+over the declare feature of TypeScript:
+
+```ts
+import { Controller } from '@hotwired/stimulus';
+
+export default class MyController extends Controller {
+    static targets = [
+        'input'
+    ];
+
+    declare readonly inputTarget: HTMLInputElement;
+}
+```
+
+> The `declare` avoids override of the exist stimulus property and just define the type for TypeScript.
+
+## Custom properties amd methods
+
+Other custom properties is possible to define the TypeScript way
+on the controller class:
+
+```ts
+import { Controller } from '@hotwired/stimulus';
+
+export default class MyController extends Controller {
+    container: HTMLElement;
+}
+```
+
+Read more about it in the [TypeScript Documentation](https://www.typescriptlang.org/docs/handbook/intro.html).