Explicitly support video (#1660)

Adds documentation on how to use videos and a little style.

Closes #995

Author: nik9000

README.asciidoc

@@ -1674,6 +1674,24 @@ Which looks like:
 
 image::resources/readme/example.svg[An example svg]
 
+[[videos]]
+== Videos
+
+You can add a vimeo hosted video with Asciidoctor's
+https://asciidoctor.org/docs/user-manual/#video[video] tag:
+
+[source,asciidoc]
+----
+video::366852847[vimeo,height=480]
+----
+
+NOTE: You *should* set height or else the video will be tiny. You *shouldn't*
+      set width because Vimeo will preserve the aspect ratio for you.
+
+Which renders like this:
+
+video::366852847[vimeo,height=480]
+
 [[tables]]
 == Tables
 

resources/web/style/video.pcss

@@ -0,0 +1,11 @@
+#guide {
+  /* This should line up with img.pcss. */
+  .videoblock {
+    img {
+      max-width: 100%;
+    }
+    iframe {
+      width: 100%
+    }
+  }
+}

resources/web/styles.pcss

@@ -26,6 +26,7 @@
 @import './style/this_page.pcss';
 @import './style/toc.pcss';
 @import './style/util.pcss';
+@import './style/video.pcss';
 @import './style/xpack.pcss';
 
 /* test comment used to detect unminified source */