Document babel helpers (#6364) [skip ci]

packages/babel-helpers/README.md

@@ -10,6 +10,8 @@ npm install --save-dev babel-helpers
 
 ## Usage
 
+Direct:
+
 ```js
 import * as helpers from 'babel-helpers';
 import * as t from 'babel-types';
@@ -19,3 +21,36 @@ const typeofHelper = helpers.get('typeof');
 t.isExpressionStatement(typeofHelper);
 // true
 ```
+
+Inside a plugin:
+
+```js
+export default {
+  visitor: {
+    UnaryExpression(path) {
+      // The .addHelper function adds, if needed, the helper to the file
+      // and returns an expression which references the helper
+      const typeofHelper = this.addHelper("typeof");
+      t.isExpression(typeofHelper); // true
+  }
+};
+```
+
+## Defining Helpers
+
+> **NOTE**: This package is only meant to be used by the packages inluded in this repository. There is currently no way for third-party plugins to define an helper.
+
+Helpers are defined in the `src/helpers.js` file, and they must be valid modules which follow these guidelines:
+ - They must have a default export, which is their entry-point.
+ - They can import other helpers, exclusively by using default imports.
+ - They can't have named exports.
+ 
+```js
+helpers.customHelper = defineHelper(`
+  import dep from "dependency";
+  const foo = 2;
+  export default function getFooTimesDepPlusX(x) {
+    return foo * dep() + x;
+  }
+`);
+```