README.md in random-port-0.3.0 vs README.md in random-port-0.3.1

@@ -2,10 +2,11 @@
 [![We recommend RubyMine](http://www.elegantobjects.org/rubymine.svg)](https://www.jetbrains.com/ruby/)
 
 [![Build Status](https://travis-ci.org/yegor256/random-port.svg)](https://travis-ci.org/yegor256/random-port)
 [![Gem Version](https://badge.fury.io/rb/random-port.svg)](http://badge.fury.io/rb/random-port)
 [![Maintainability](https://api.codeclimate.com/v1/badges/349b8c31884d3b34d926/maintainability)](https://codeclimate.com/github/yegor256/random-port/maintainability)
+[![Yard Docs](http://img.shields.io/badge/yard-docs-blue.svg)](http://rubydoc.info/github/yegor256/random-port/master/frames)
 
 It's a simple Ruby gem to get a random TCP port.
 
 First, install it:
 
@@ -44,10 +45,12 @@
 RandomPort::Pool::SINGLETON.acquire do |port|
   # Use it here...
 end
 ```
 
-The pool is _not_ thread-safe.
+The pool is thread-safe by default.
+You can configure it to be
+not-thread-safe, using optional `sync` argument of the constructor.
 
 # How to contribute
 
 Read [these guidelines](https://www.yegor256.com/2014/04/15/github-guidelines.html).
 Make sure you build is green before you contribute