Add qualification status for supported platforms in the README

We've been able to build for more platforms in our CI, so update the
README to reflect this triumph. Explain how the different qualification
levels differ, and focus on build + test for all platforms in the future.

Move some platforms from Experimental -> Supported, although not with
full CI build + test yet.

Author: jerryjrchen

README.md

@@ -106,14 +106,23 @@ Swift.
 The table below describes the current level of support that Swift Testing has
 for various platforms:
 
-| **Platform** | **Support Status** |
-|-|-|
-| Apple platforms | Supported |
-| Linux | Supported |
-| Windows | Supported |
-| FreeBSD, OpenBSD | Experimental |
-| Wasm | Experimental |
-| Android | Experimental |
+| **Platform**     | **Support Status** | **Qualification[^1]**        |
+| ---------------- | ------------------ | ---------------------------- |
+| Apple platforms  | Supported          | Automated CI: Build and Test |
+| Linux            | Supported          | Automated CI: Build and Test |
+| Windows          | Supported          | Automated CI: Build and Test |
+| Wasm             | Experimental       | Automated CI: Build          |
+| Android          | Experimental       | Automated CI: Build          |
+| FreeBSD, OpenBSD | Experimental       | Manual                       |
+
+[^1]:
+    Most platforms have "Automated CI: Build and Test" qualification, where
+    continuous integration automatically verifies that the project builds and
+    passes all tests. This ensures that any changes meet our highest quality
+    standards, so it is our goal for all supported platforms.
+
+    Presently, some platforms rely on manual test ("Automated Build"
+    qualification) or manual build and test ("Manual" qualification).
 
 ### Works with XCTest
 
@@ -127,7 +136,7 @@ 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.  You do not need to add it as a package dependency to
+toolchain and Xcode 16. You do not need to add it as a package dependency to
 your Swift package or Xcode project.
 
 > [!IMPORTANT]
@@ -136,5 +145,5 @@ your Swift package or Xcode project.
 > 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) 
+[README](https://github.com/swiftlang/swift-testing/blob/main/Documentation/README.md)
 of the `Documentation/` subdirectory.