Fix README and updated gitignore

[fbjorn]

I tried to debug Web UI locally, but realised that instructions had a typo.
Also added IDEA to gitignore for anyone using JetBrains IDEs

[fbjorn]

Oh BTW I wasn't able to follow the instructions in the end. Because you need to configure CORS on "main" backend for that, and I didn't understand how should I modify aw-server.toml in my installed application (I'm on macOS)

Instead, I ran another server and it worked:

AW_SERVER_URL="'http://localhost:5601'" npm run build -- --watch --dest=../aw_server/static
# in another tab:
cd ..
poetry run aw-server --port 5601

Not sure this method worth mentioning and how correct it is