[guide] Added documentation for arrow function parens with one arg

Added explanation for using parentheses with explicit returns and alternatives with the eslint "always" option.

Author: ericmarkmartin

README.md

@@ -925,7 +925,7 @@ Other Style Guides
     ```
 
   <a name="arrows--one-arg-parens"></a><a name="8.4"></a>
-  - [8.4](#arrows--one-arg-parens) If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. eslint: [`arrow-parens`](http://eslint.org/docs/rules/arrow-parens.html) jscs:  [`disallowParenthesesAroundArrowParam`](http://jscs.info/rule/disallowParenthesesAroundArrowParam)
+  - [8.4](#arrows--one-arg-parens) If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments for clarity and consistency. Note: it is also acceptable to always use parentheses, in which case use the ["always" option](http://eslint.org/docs/rules/arrow-parens#always) for eslint or do not include [`disallowParenthesesAroundArrowParam`](http://jscs.info/rule/disallowParenthesesAroundArrowParam) for jscs. eslint: [`arrow-parens`](http://eslint.org/docs/rules/arrow-parens.html) jscs:  [`disallowParenthesesAroundArrowParam`](http://jscs.info/rule/disallowParenthesesAroundArrowParam)
 
     > Why? Less visual clutter.