[eslint config] [breaking] require outer IIFE wrapping; flesh out guide section.

There was lots of discussion [here](airbnb#21 (comment)), but now that we have both a modern build system and an eslint rule requiring terminating semicolons, the concerns with the “crockford” style no longer apply.

Author: ljharb

README.md

@@ -506,13 +506,17 @@ Other Style Guides
     }
     ```
 
-  - [7.2](#7.2) <a name='7.2'></a> Function expressions:
+  - [7.2](#7.2) <a name='7.2'></a> Immediately invoked function expressions:
+
+    > Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.
+
+    eslint rules: [`wrap-iife`](http://eslint.org/docs/rules/wrap-iife.html).
 
     ```javascript
     // immediately-invoked function expression (IIFE)
-    (() => {
+    (function () {
       console.log('Welcome to the Internet. Please follow me.');
-    })();
+    }());
     ```
 
   - [7.3](#7.3) <a name='7.3'></a> Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears.

packages/eslint-config-airbnb/rules/best-practices.js

@@ -108,7 +108,8 @@ module.exports = {
     // requires to declare all vars on top of their containing scope
     'vars-on-top': 2,
     // require immediate function invocation to be wrapped in parentheses
-    'wrap-iife': [2, 'any'],
+    // http://eslint.org/docs/rules/wrap-iife.html
+    'wrap-iife': [2, 'outside'],
     // require or disallow Yoda conditions
     'yoda': 2
   }