[guide] Attempt to clarify filename conventions

Fixes airbnb#817.

README.md

@@ -2337,24 +2337,39 @@ Other Style Guides
     ```
 
   <a name="naming--filename-matches-export"></a><a name="22.6"></a>
-  - [22.6](#naming--filename-matches-export) If your file exports a single class, your filename should be exactly the name of the class.
+  - [22.6](#naming--filename-matches-export) A base filename should exactly match the name of its default export.
 
     ```javascript
-    // file contents
+    // file 1 contents
     class CheckBox {
       // ...
     }
     export default CheckBox;
 
+    // file 2 contents
+    export default function fortyTwo() { return 42; }
+
+    // file 3 contents
+    export default function insideDirectory() {}
+
     // in some other file
     // bad
-    import CheckBox from './checkBox';
+    import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
+    import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
+    import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export
 
     // bad
-    import CheckBox from './check_box';
+    import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
+    import forty_two from './forty_two'; // snake_case import/filename, camelCase export
+    import inside_directory from './inside_directory'; // snake_case import, camelCase export
+    import index from './inside_directory/index'; // requiring the index file explicitly
+    import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly
 
     // good
-    import CheckBox from './CheckBox';
+    import CheckBox from './CheckBox'; // PascalCase export/import/filename
+    import fortyTwo from './fortyTwo'; // camelCase export/import/filename
+    import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
+    // ^ supports both insideDirectory.js and insideDirectory/index.js
     ```
 
   <a name="naming--camelCase-default-export"></a><a name="22.7"></a>
@@ -2368,7 +2383,7 @@ Other Style Guides
     ```
 
   <a name="naming--PascalCase-singleton"></a><a name="22.8"></a>
-  - [22.8](#naming--PascalCase-singleton) Use PascalCase when you export a singleton / function library / bare object.
+  - [22.8](#naming--PascalCase-singleton) Use PascalCase when you export a constructor / class / singleton / function library / bare object.
 
     ```javascript
     const AirbnbStyleGuide = {