You can start reading here :)

P.S. If your code still relies on the old synchronous behaviour, don't worry, there is backward compatibility.

However, old synchronous behaviour is deprecated, and I highly recommend you switch to async, due to the huge performance and parallelism benefits.

In a few weeks I will set MadelineProto 4.0 as default with madeline.php: in the meantime, I do not provide support for the old version.