docs(generators:decorator): create decorator.md

[skip ci]

Author: Awk34

docs/generators/decorator.md

@@ -0,0 +1,12 @@
+### Decorator
+Generates an AngularJS service decorator.
+
+Example:
+```bash
+yo angular-fullstack:decorator serviceName
+[?] Where would you like to create this decorator? client/app/
+```
+
+Produces
+
+    client/app/serviceName/serviceName.decorator.js