add docs: whats new entry

docs/source/whatsnew/pr/enable-to-add-extra-attrs-to-iframe.rst

@@ -0,0 +1,37 b''
+Enable to add extra attributes to iframe
+========================================
+
+You can add any extra attributes to the ``<iframe>`` tag
+since the argument ``extras`` has been added to the ``IFrame`` class.
+for example::
+
+    In [1]: from IPython.display import IFrame
+
+    In [2]: print(IFrame(src="src", width=300, height=300, extras=["hello", "world"])._repr_html_())
+
+            <iframe
+                width="300"
+                height="300"
+                src="src"
+                frameborder="0"
+                allowfullscreen
+                hello world
+            ></iframe>
+
+Using it, you can autoplay ``YouTubeVideo`` by adding ``'allow="autoplay"'``,
+even in browsers that disable it by default, such as Google Chrome.
+And, you can write it more briefly by using the argument ``allow_autoplay``.
+::
+
+    In [1]: from IPython.display import YouTubeVideo
+
+    In [2]: print(YouTubeVideo("video-id", allow_autoplay=True)._repr_html_())
+
+            <iframe
+                width="400"
+                height="300"
+                src="https://www.youtube.com/embed/video-id?autoplay=1"
+                frameborder="0"
+                allowfullscreen
+                allow="autoplay"
+            ></iframe>