Explain how to configure timeout in Readme

[mtangoo]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues	#40

[what-the-diff]

PR Summary

• Documentation update for migrating from older Swiftmailer
  This change provides essential updates and clarifications in the documentation about migrating from previous versions of swiftmailer. It specifically mentions that the default mailing system, referred to as 'transport' here, is initially set to a dummy one, which sends no emails. Additionally, it includes an example demonstrating how to set up the default mailing system in the application's configuration.

• Addition of a custom SMTP factory class example in the documentation
  The documentation now includes an example of how to create and use a custom class for setting up SMTP, a widespread method for sending emails. This class also shows how to adjust the timeout parameter, a feature not directly supported by this extension due to restrictions imposed by the underlying software component, 'Symfony'.

[samdark]

Thanks!