Update client authentication documentation

The changes simplify and clarify how client authentication works, making
it clearer that auth happens automatically on client creation rather
than lazily on first API call.

Author: bruno-costanzo

README.md

@@ -65,32 +65,21 @@ Fripa.config.verify_ssl = false
 
 ### Creating a Client
 
-Create a client instance with user credentials. Each client maintains its own session:
+Create a client instance with user credentials. The client authenticates immediately upon creation:
 
 ```ruby
 client = Fripa::Client.new(
   username: 'your-username',
   password: 'your-password'
 )
+# Client is now authenticated and ready to use
 ```
 
-The client will automatically authenticate when making API calls.
-
-### Manual Authentication (Optional)
-
-If you need to authenticate manually:
-
-```ruby
-client.authenticator.login!
-# The session cookie is now stored in the client
-puts client.session_cookie
-```
-
-If authentication fails, a `Fripa::AuthenticationError` will be raised:
+If authentication fails, a `Fripa::AuthenticationError` will be raised immediately:
 
 ```ruby
 begin
-  client.authenticator.login!
+  client = Fripa::Client.new(username: 'admin', password: 'wrong-password')
 rescue Fripa::AuthenticationError => e
   puts "Authentication failed: #{e.message}"
 end