Add Periphery as an example.

Author: ileitch

README.md

@@ -47,6 +47,8 @@ Then, import `SwiftSyntax` in your Swift code.
 
 [**SwiftPack**](https://github.com/omochi/SwiftPack): a tool for automatically embedding Swift library source.
 
+[**Periphery**](https://github.com/peripheryapp/periphery): a tool to detect unused code analyzer.
+
 ## Contributing
 
 ### Building SwiftSyntax from `master`
@@ -61,26 +63,26 @@ To build the `master` version of SwiftSyntax, check `swift-syntax` and  `swift`
 
 And run `swift-syntax/build-script.py`. SwiftSyntax is now being built with the Swift compiler installed on the system.
 
-Swift-CI will automatically run the code generation step whenever a new toolchain (development snapshot or release) is published. It should thus almost never be necessary to perform the above build yourself. 
+Swift-CI will automatically run the code generation step whenever a new toolchain (development snapshot or release) is published. It should thus almost never be necessary to perform the above build yourself.
 
 Afterwards, SwiftPM can also generate an Xcode project to develop SwiftSyntax by running `swift package generate-xcodeproj`.
 
-If you also want to run tests locally, read the section below as testing has additional requirements. 
+If you also want to run tests locally, read the section below as testing has additional requirements.
 
 ### Local Testing
-SwiftSyntax uses some test utilities that need to be built as part of the Swift compiler project. To build the most recent version of SwiftSyntax and test it, follow the steps in [swift/README.md](https://github.com/apple/swift/blob/master/README.md) and pass `--llbuild --swiftpm --swiftsyntax` to the build script invocation to build SwiftSyntax and all its dependencies using the current `master` compiler. 
+SwiftSyntax uses some test utilities that need to be built as part of the Swift compiler project. To build the most recent version of SwiftSyntax and test it, follow the steps in [swift/README.md](https://github.com/apple/swift/blob/master/README.md) and pass `--llbuild --swiftpm --swiftsyntax` to the build script invocation to build SwiftSyntax and all its dependencies using the current `master` compiler.
 
-SwiftSyntax can then be tested using the build script in `apple/swift` by running 
+SwiftSyntax can then be tested using the build script in `apple/swift` by running
 ```
 swift/utils/build-script --swiftsyntax --swiftpm --llbuild -t --skip-test-cmark --skip-test-swift --skip-test-llbuild --skip-test-swiftpm
 ```
-This command will build SwiftSyntax and all its dependencies, tell the build script to run tests, but skip all tests but the SwiftSyntax tests. 
+This command will build SwiftSyntax and all its dependencies, tell the build script to run tests, but skip all tests but the SwiftSyntax tests.
 
 Note that it is not currently supported to SwiftSyntax while building the Swift compiler using Xcode.
 
 ### CI Testing
 
-Running `@swift-ci Please test` on the main Swift repository will also test the most recent version of SwiftSyntax. 
+Running `@swift-ci Please test` on the main Swift repository will also test the most recent version of SwiftSyntax.
 
 Testing SwiftSyntax from its own repository is now available by commenting `@swift-ci Please test macOS platform`.