[guide] Added 'let' to rule defs regarding variables

Author: flyingace

README.md

@@ -1450,7 +1450,7 @@ Other Style Guides
 ## Variables
 
   <a name="variables--const"></a><a name="13.1"></a>
-  - [13.1](#variables--const) Always use `const` to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: [`no-undef`](http://eslint.org/docs/rules/no-undef) [`prefer-const`](http://eslint.org/docs/rules/prefer-const)
+  - [13.1](#variables--const) Always use `const` or `let` to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: [`no-undef`](http://eslint.org/docs/rules/no-undef) [`prefer-const`](http://eslint.org/docs/rules/prefer-const)
 
     ```javascript
     // bad
@@ -1461,7 +1461,7 @@ Other Style Guides
     ```
 
   <a name="variables--one-const"></a><a name="13.2"></a>
-  - [13.2](#variables--one-const) Use one `const` declaration per variable. eslint: [`one-var`](http://eslint.org/docs/rules/one-var.html) jscs: [`disallowMultipleVarDecl`](http://jscs.info/rule/disallowMultipleVarDecl)
+  - [13.2](#variables--one-const) Use one `const` or `let` declaration per variable. eslint: [`one-var`](http://eslint.org/docs/rules/one-var.html) jscs: [`disallowMultipleVarDecl`](http://jscs.info/rule/disallowMultipleVarDecl)
 
     > Why? It's easier to add new variable declarations this way, and you never have to worry about swapping out a `;` for a `,` or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.