docs(cheatsheet): add Directive to cheatsheet

Closes #8170

modules/angular2/docs/cheatsheet/class-decorators.md

@@ -18,6 +18,18 @@ class MyComponent() {}`|`@Component(...)`
 description:
 Declares that a class is a component and provides metadata about the component.
 
+@cheatsheetItem
+syntax(ts):
+`@Directive({...})
+class MyDirective() {}`|`@Directive({...})`
+syntax(js):
+`var MyDirective = ng.core.Directive({...}).Class({...})`|`ng.core.Directive({...})`
+syntax(dart):
+`@Directive(...)
+class MyDirective() {}`|`@Directive(...)`
+description:
+Declares that a class is a directive and provides metadata about the directive.
+
 @cheatsheetItem
 syntax(ts):
 `@Pipe({...})