docs: Add example for .response() also to API section

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Author: deathbeam

README.md

@@ -167,6 +167,9 @@ chat.ask("Show me something interesting", {
 -- Get all available prompts (can be used for integrations like fzf/telescope)
 local prompts = chat.prompts()
 
+-- Get last copilot response (also can be used for integrations and custom keymaps)
+local response = chat.response()
+
 -- Pick a prompt using vim.ui.select
 local actions = require("CopilotChat.actions")