Pls update Readme file to reflect the newest changes in `if let` syntax introduced in Swift 5.7

[Krzysztof Pacholak]

This new syntax comes with this change
https://github.com/apple/swift-evolution/blob/main/proposals/0345-if-let-shorthand.md

We can add to the Readme Optionals section something like this:

Notes: Swift 5.7 introduced new shorthand syntax for unwrapping optionals into shadowed variables:

if let textContainer {
  // do many things with textContainer
}

[Krzysztof Pacholak]

I've already created a PR
#359