title: what is ngrx createSelector and createFeatureSelector?
body: <p>I have been reading the code of ngrx example app and find two function calls  </p>
  
  <ol>
  <li><code>createFeatureSelector&lt;AuthState&gt;('auth');</code>   </li>
  </ol>
  
  <p>and</p>
  
  <ol start="2">
  <li><code>createSelector(selectAuthState,(state: AuthState) =&gt; state.status);</code></li>
  </ol>
  
  <p>What does this do?</p>
  
  <pre><code>export const selectAuthState = createFeatureSelector&lt;AuthState&gt;('auth');
  
  export const selectAuthStatusState = createSelector(
    selectAuthState,
    (state: AuthState) =&gt; state.status
  );
  </code></pre>