docs: add Atomic Chat standalone integration to README

[yanalialiuk]

Proposal

Add an Atomic Chat section to the README (branch ready: https://github.com/yanalialiuk/pool/tree/docs/atomic-chat-integration).

Atomic Chat runs a local OpenAI-compatible API on port 1337:

POOLSIDE_STANDALONE_BASE_URL="http://127.0.0.1:1337/v1" \
POOLSIDE_API_KEY="atomic-chat-local" \
pool -m <model-id>

Compare / open PR: main...yanalialiuk:pool:docs/atomic-chat-integration