[guide] [breaking] Disallow leading underscores - there’s no such thi…

…ng as “private properties”.

Per airbnb#490 (comment)

README.md

@@ -2294,15 +2294,18 @@ Other Style Guides
     ```
 
   <a name="naming--leading-underscore"></a><a name="22.4"></a>
-  - [22.4](#naming--leading-underscore) Use a leading underscore `_` when naming private properties. eslint: [`no-underscore-dangle`](http://eslint.org/docs/rules/no-underscore-dangle.html) jscs: [`disallowDanglingUnderscores`](http://jscs.info/rule/disallowDanglingUnderscores)
+  - [22.4](#naming--leading-underscore) Do not use trailing or leading underscores. eslint: [`no-underscore-dangle`](http://eslint.org/docs/rules/no-underscore-dangle.html) jscs: [`disallowDanglingUnderscores`](http://jscs.info/rule/disallowDanglingUnderscores)
+
+    > Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won't count as breaking, or that tests aren't needed. tl;dr: if you want something to be “private”, it must not be observably present.
 
     ```javascript
     // bad
     this.__firstName__ = 'Panda';
     this.firstName_ = 'Panda';
+    this._firstName = 'Panda';
 
     // good
-    this._firstName = 'Panda';
+    this.firstName = 'Panda';
     ```
 
   <a name="naming--self-this"></a><a name="22.5"></a>