title: Why `NotificationCenter+Rx` is in RxCocoa not RxSwift
body: <p>Since <a href="https://developer.apple.com/documentation/foundation/nsnotificationcenter" rel="nofollow noreferrer">NotificationCenter</a> is a part of <a href="https://developer.apple.com/documentation/foundation" rel="nofollow noreferrer">Foundation</a>. I expect it should be within RxSwift, not RxCocoa.</p>
  
  <p>But why <a href="https://github.com/ReactiveX/RxSwift/blob/f043778214c8f182018ccdfbf7f440edbe0aecc8/RxCocoa/Foundation/NotificationCenter%2BRx.swift" rel="nofollow noreferrer">NotificationCenter+Rx</a> is in RxCocoa? Any reason behind that?</p>
  
  <p>In some of our project, we only use RxSwift without RxCocoa. If we use <code>NotificationCenter.default.rx.notification</code>, we will get the error <code>Value of type 'Reactive&lt;NotificationCenter&gt;' has no member 'notification'</code> because NotificationCenter+Rx is in RxCocoa only.</p>