Add short option and docs

Author: keith

README.md

@@ -82,6 +82,9 @@ as well as the following command line options:
 * `-i/--in-place`: Overwrites the input files when formatting instead of
   printing the results to standard output.
 
+* `-p/--parallel`: Whether or not to run formatting or linting in parallel
+  consuming all possible resources printing the results to standard output.
+
 * `-r/--recursive`: If specified, then the tool will process `.swift` source
   files in any directories listed on the command line and their descendants.
   Without this flag, it is an error to list a directory on the command line.

Sources/swift-format/Subcommands/LintFormatOptions.swift

@@ -50,7 +50,9 @@ struct LintFormatOptions: ParsableArguments {
   var ignoreUnparsableFiles: Bool = false
 
   /// Whether or not to run the formatter/linter in parallel.
-  @Flag(help: "Process files in parallel, simultaneously across multiple cores.")
+  @Flag(
+    name: .shortAndLong,
+    help: "Process files in parallel, simultaneously across multiple cores.")
   var parallel: Bool = false
 
   /// The list of paths to Swift source files that should be formatted or linted.