doc: add README usage guide

Author: nfischer

README.md

@@ -21,3 +21,31 @@ appropriate for the language of the script, ex. `coffee`).
 
 This package was removed from ShellJS to reduce that module's dependencies and
 to shrink overall download size.
+
+## Usage
+
+You can install with `npm install shelljs-shjs`:
+
+```shell
+$ npm install -g shelljs-shjs
+```
+
+This script is a convenience wrapper for guessing the right interpreter to use
+for your script. It also brings your local shelljs install to the front of the
+`NODE_PATH` to ensure this is used.
+
+```javascript
+#!/usr/bin/env shjs
+
+// Save this demo as 'example_script.js'
+
+var shell = require('shelljs');
+shell.echo('hello world');
+```
+
+Then run the demo script with:
+
+```shell
+$ shjs example_script.js
+hello world
+```

package-lock.json

@@ -41,8 +41,8 @@
   },
   "dependencies": {
     "interpret": "^1.2.0",
-    "shelljs": "^0.9.2",
-    "rechoir": "^0.7.0"
+    "rechoir": "^0.7.0",
+    "shelljs": "^0.9.2"
   },
   "devDependencies": {
     "ava": "^6.2.0",