docs: cleanup docs around shell defaults and supported

src/index.ts

@@ -92,7 +92,7 @@ function getExecutable(): string {
       break;
     }
     default: {
-      throw new Error(`Shell ${SHELL} required`);
+      throw new Error(`Shell ${SHELL} not supported.  See https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-syntax-for-github-actions#using-a-specific-shell for supported shells`);
     }
   }
   return executable