add part about testing frontend & backend using local server

README.md

@@ -217,6 +217,24 @@ URLs for development:
 * `localhost:8080/last-comments.html` — page with embedded script for last comments;
 * `localhost:8080/counter.html` — page with embedded script for counter with examples.
 
+#### Testing
+
+Also you can use fully functional local version to develop and test both frontend & backend.
+
+To bring it up run:
+
+```bash
+docker-compose -f compose-dev.yml build
+docker-compose -f compose-dev.yml up
+```
+
+It starts Remark42 on `localhost:8080` 
+and adds local OAuth2 provider “Dev”. To access UI demo page go to `localhost:8080/web`.
+
+That `compose-dev.yml` (you can find it in the root of the project)
+also defines if logged user admin or not. 
+By default, it will be the admin, and to switch it to regular user comment or remove `-ADMIN=dev_user` there.
+
 #### Usage
 
 ##### Comments