Merge branch 'master' into argumentparser

Author: natecook1000

README.md

@@ -33,6 +33,7 @@ build `swift-format` using the following commands:
 
 ```
 git clone -b swift-5.1-branch https://github.com/apple/swift-format.git
+cd swift-format
 swift build
 ```
 

Sources/swift-format/Run.swift

@@ -76,6 +76,9 @@ func formatMain(
   configuration: Configuration, sourceFile: FileHandle, assumingFilename: String?, inPlace: Bool,
   debugOptions: DebugOptions, diagnosticEngine: DiagnosticEngine
 ) {
+  // Even though `diagnosticEngine` is defined, it's use is reserved for fatal messages. Pass nil
+  // to the formatter to suppress other messages since they will be fixed or can't be automatically
+  // fixed anyway.
   let formatter = SwiftFormatter(configuration: configuration, diagnosticEngine: nil)
   formatter.debugOptions = debugOptions
   let assumingFileURL = URL(fileURLWithPath: assumingFilename ?? "<stdin>")