README.md in random-port-0.2.0 vs README.md in random-port-0.3.0

@@ -44,10 +44,24 @@
 RandomPort::Pool::SINGLETON.acquire do |port|
   # Use it here...
 end
 ```
 
-The pool is thread-safe.
+The pool is _not_ thread-safe.
+
+# 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
+your pull request. You will need to have [Ruby](https://www.ruby-lang.org/en/) 2.3+ and
+[Bundler](https://bundler.io/) installed. Then:
+
+```
+$ bundle update
+$ rake
+```
+
+If it's clean and you don't see any error messages, submit your pull request.
 
 # License
 
 (The MIT License)