Add note to explain what a soft tab is

A quick Google search shows that the name "soft tab" causes some confusion. Added a brief clarification.

Author: joshduck

README.md

@@ -2068,7 +2068,7 @@ Other Style Guides
 ## Whitespace
 
  <a name="whitespace--spaces"></a><a name="18.1"></a>
- - [18.1](#whitespace--spaces) Use soft tabs set to 2 spaces. eslint: [`indent`](http://eslint.org/docs/rules/indent.html) jscs: [`validateIndentation`](http://jscs.info/rule/validateIndentation)
+ - [18.1](#whitespace--spaces) Use soft tabs (space character) set to 2 spaces. eslint: [`indent`](http://eslint.org/docs/rules/indent.html) jscs: [`validateIndentation`](http://jscs.info/rule/validateIndentation)
 
  ```javascript
  // bad