doc: add information how to open chat to README file (#178)

README.md

@@ -66,6 +66,10 @@ After installation and configuration, you will need to authenticate with
 Codeium. This can be done by running `:Codeium Auth`, copying the token from
 your browser and pasting it into API token request.
 
+To use Codeium Chat, execute the `:Codeium Chat` command (this feature is only
+available when chat is enabled in the [configuration](#options)). The chat will be opened
+in your default browser using the xdg-open command.
+
 ## Options
 
 - `config_path`: the path to the config file, used to store the API key.