title: What is the purpose of ngrx/effects library?
body: <p>I have failed to find any usefull information about this library or what is the purpose of it.
  It seems like <a href="https://github.com/ngrx/effects" rel="noreferrer">ngrx/effects</a> explain this library to developers who already know this concept and gives a bigginer example on how to code.</p>
  <p><strong>My questions:</strong></p>
  <ol>
  <li>What are sources of actions?</li>
  <li>What is the purpose of ngrx/effects library;what is the downside of only using ngrx/store?</li>
  <li>When it is recommended to be used?</li>
  <li>Does it support angular rc 5+? How do we configure it in rc 5+?</li>
  </ol>
  <p>Thanks!</p>