Rename 'archs' input and improve multiarch (#62)

- Expand readme
- Add example

Signed-off-by: Tim Etchells <tetchel@gmail.com>

src/generated/inputs-outputs.ts

@@ -1,10 +1,15 @@
 // This file was auto-generated by action-io-generator. Do not edit by hand!
 export enum Inputs {
     /**
-     * Architecture(s) to build the image(s) for. For multiple architectures,
-     * separate by a comma.
+     * Label the image with this ARCH, instead of defaulting to the host architecture.
      * Required: false
-     * Default: "amd64"
+     * Default: None.
+     */
+    ARCH = "arch",
+    /**
+     * Alias for "arch". "arch" takes precedence if both are set.
+     * Required: false
+     * Default: None.
      */
     ARCHS = "archs",
     /**