Added basic docs

Author: samdeane

README.md

@@ -1,3 +1,19 @@
 # SwiftFormatterPlugin
 
-A description of this package.
+A Swift Package Manager plugin which runs [SwiftFormat](https://github.com/nicklockwood/SwiftFormat).
+
+## Usage
+
+Add this repo to your package dependencies:
+
+```Swift
+    dependencies: [
+        .package(url: "https://github.com/elegantchaos/SwiftFormatterPlugin", from: "1.0.2"),
+        /* other dependencies here... */ 
+    ],
+```
+
+Invoke the tool from the command line:
+
+`swift package plugin --allow-writing-to-package-directory format-source-code`
+