Update README.md's note about development toolchain requirements. (#911)

We are well past Swift 6.0 development toolchains. Update the readme to
reflect that.

Resolves #846.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

README.md

@@ -112,18 +112,17 @@ incrementally, at your own pace.
 
 ## Documentation
 
-> [!IMPORTANT]
-> This package is under active, ongoing development and requires a recent
-> **6.0 development snapshot** toolchain. Its contents and interfaces are still
-> considered experimental at this time and may change. See this
-> [Forum post](https://forums.swift.org/t/an-update-on-swift-testing-progress-and-stable-release-plans/71455)
-> for details about stable release plans.
-
 Detailed documentation for Swift Testing can be found on the
 [Swift Package Index](https://swiftpackageindex.com/swiftlang/swift-testing/main/documentation/testing).
 There, you can delve into comprehensive guides, tutorials, and API references to
 make the most out of this package. Swift Testing is included with the Swift 6
-toolchain and Xcode 16.
+toolchain and Xcode 16.  You do not need to add it as a package dependency to
+your Swift package or Xcode project.
+
+> [!IMPORTANT]
+> Swift Testing depends on upcoming language and compiler features. If you are
+> building Swift Testing from source, be aware that the main branch of this
+> repository requires a recent **main-branch development snapshot** toolchain.
 
 Other documentation resources for this project can be found in the
 [README](https://github.com/swiftlang/swift-testing/blob/main/Documentation/README.md)