Merge pull request #259 from allevato/prep-5.5

Add Xcode 13.0 to the README.

Author: allevato

README.md

@@ -26,6 +26,7 @@ is also expressed in the `SwiftSyntax` dependency in
 | Xcode Release | Swift Version                           | `swift-format` Branch |
 |:-------------:|:---------------------------------------:|:----------------------|
 | –             | Swift at `main`                         | `main`                |
+| Xcode 13.0    | Swift 5.5                               | `swift-5.5-branch`    |
 | Xcode 12.5    | Swift 5.4                               | `swift-5.4-branch`    |
 | Xcode 12.0    | Swift 5.3                               | `swift-5.3-branch`    |
 | Xcode 11.4    | Swift 5.2                               | `swift-5.2-branch`    |