Fix: Add note about posts directory in context of custom collections …

…directory

docs/_docs/collections.md

@@ -55,6 +55,12 @@ defaults:
   in <code>my_collections/_recipes</code> for the <code>recipes</code> collection.</p>
 </div>
 
+<div class="note warning">
+  <h5>Be sure to move posts into custom collections directory</h5>
+
+  <p>If you specify a directory to store all your collections in the same place with <code>collections_dir: my_collections</code>, then you will need to move your <code>_posts</code> directory to <code>my_collections/_post</code>.</p>
+</div>
+
 ### Step 2: Add your content {#step2}
 
 Create a corresponding folder (e.g. `<source>/_my_collection`) and add