feat(engine-handlebars): Document the Helpers feature

Author: engelfrost

packages/engine-handlebars/README.md

@@ -1,6 +1,6 @@
 # The Handlebars PatternEngine for Pattern Lab / Node
 
-To install the Handlebars PatternEngine in your edition, `npm install @pattern-lab/engine-handlebars` should do the trick.
+To install the Handlebars PatternEngine in your edition, `npm install --save @pattern-lab/engine-handlebars` should do the trick.
 
 ## Supported features
 
@@ -11,3 +11,34 @@ To install the Handlebars PatternEngine in your edition, `npm install @pattern-l
 * [x] [Pattern States](http://patternlab.io/docs/pattern-states.html)
 * [ ] [Pattern Parameters](http://patternlab.io/docs/pattern-parameters.html) (Accomplished instead using [native Handlebars partial arguments](http://handlebarsjs.com/partials.html))
 * [ ] [Style Modifiers](http://patternlab.io/docs/pattern-stylemodifier.html) (Accomplished instead using [native Handlebars partial arguments](http://handlebarsjs.com/partials.html))
+
+## Helpers
+
+To add custom [helpers](http://handlebarsjs.com/#helpers) or otherwise interact with Handlebars directly, create a file named `patternlab-handlebars-config.js` in the root of your Pattern Lab project, or override the default location by specifying one or several glob patterns in the Pattern Lab config:
+
+```json
+  {
+    ...
+    "engines": {
+      "handlebars": {
+        "helpers": [
+          "handlebars-helpers.js",
+          "helpers/**/*.js"
+        ]
+      }
+    }
+  }
+```
+
+Each file should export a function which takes Handlebars as an argument.
+
+```js
+module.exports = function(Handlebars) {
+  // Put helpers here
+
+  Handlebars.registerHelper('fullName', function(person) {
+    // Example: person = {firstName: "Alan", lastName: "Johnson"}
+    return person.firstName + " " + person.lastName;
+  });
+};
+```