add testing info to README

README.md

 The js script can be introduced on any website and it will inject a chat bubble.
 The backend repo can be found at https://gitlab.cs.pub.ro/netsys/js_chat_widget
+
+## Run test:
+
+For testing locally, you can serve this with python:
+
+```
+$ python3 -m http.server
+```
+
+(by default, this is port 8000)
+
+And change the URL of the backend (and of the feedback and report endpoints):
+
+
+In `js/api.js`, change:
+
+```
+- const BACKEND_URL = 'https://chat.upb.ro/api/chat/';
++ const BACKEND_URL = 'http://localhost:8888/api/chat/';
+```
+
+In `js/report.js`, change:
+
+```
+- const REPORT_URL = 'https://chat.upb.ro/api/report/';
++ const REPORT_URL = 'http://localhost:8888/api/report/';
+```
+
+In `js/feedback.js`, change:
+
+```
+- const FEEDBACK_URL = 'https://chat.upb.ro/api/feedback/';
++ const FEEDBACK_URL = 'http://localhost:8888/api/feedback/';
+```