Update the list of maintainers

Author: eregon

README.md

@@ -375,6 +375,8 @@ best practice is to depend on `concurrent-ruby` and let users to decide if they
 * [Benoit Daloze](https://github.com/eregon)
 * [Matthew Draper](https://github.com/matthewd)
 * [Rafael França](https://github.com/rafaelfranca)
+* [Charles Oliver Nutter](https://github.com/headius)
+* [Ben Sheldon](https://github.com/bensheldon)
 * [Samuel Williams](https://github.com/ioquatix)
 
 ### Special Thanks to