docs(generators:service): create service.md

[skip ci]

Author: Awk34

docs/generators/service.md

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+### Service
+Generates an AngularJS service.
+
+Example:
+```bash
+yo angular-fullstack:service myService
+[?] Where would you like to create this service? client/app/
+```
+
+Produces:
+
+    client/app/myService/myService.service.js
+    client/app/myService/myService.service.spec.js
+
+
+You can also do `yo angular-fullstack:factory` and `yo angular-fullstack:provider` for other types of services.