Mention module name and presence in toolchain in README (#1233)

This adds a mention of the testing library's module name (`import
Testing`) and its presence in officially-supported Swift toolchains in
the `README`.

### Motivation:

Some users have been confused about whether they need to declare a
package dependency on the `swift-testing` package to use Swift Testing.
It's included in Swift toolchains for officially-supported platforms, so
we generally do not recommend declaring such a package dependency, and
doing so can lead to a degraded experience with integrated tools.

### 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: stmontgomery

README.md

@@ -23,6 +23,8 @@ Swift expressions and operators, and captures the evaluated values so you can
 quickly understand what went wrong when a test fails.
 
 ```swift
+import Testing
+
 @Test func helloWorld() {
   let greeting = "Hello, world!"
   #expect(greeting == "Hello") // Expectation failed: (greeting → "Hello, world!") == "Hello"
@@ -84,11 +86,19 @@ func mentionedContinents(videoName: String) async throws {
 
 ### Cross-platform support
 
-Swift Testing works on all major platforms supported by Swift, including Apple
-platforms, Linux, and Windows, so your tests can behave more consistently when
-moving between platforms. It’s developed as open source and discussed on the
-[Swift Forums](https://forums.swift.org/c/development/swift-testing/103) so the
-very best ideas, from anywhere, can help shape the future of testing in Swift.
+Swift Testing is included in officially-supported Swift toolchains, including
+those for Apple platforms, Linux, and Windows. To use the library, import the
+`Testing` module:
+
+```swift
+import Testing
+```
+
+You don't need to declare a package dependency to use Swift Testing. It's
+developed as open source and discussed on the
+[Swift Forums](https://forums.swift.org/c/development/swift-testing/103)
+so the very best ideas, from anywhere, can help shape the future of testing in
+Swift.
 
 The table below describes the current level of support that Swift Testing has
 for various platforms: