Mention support for OpenRouter in docs

[michalbogacz]

Current readme does not mention support for OpenRouter ( https://openrouter.ai/api/v1 with sttp.ai.openai.OpenAI)

I think it would be good if documentation mentioned it explicitly from 2 reason:

• better project visibility (broader audience), for a people which looking for libs supporting OpenRouter
• when someone codes with agent (Claude, Cursor..) and picks sttp-ai, agent will immediately know OpenRouter can be used (in my case agent did some complex analysys burning tokens)