Add an example of embedding a Skia Perf graph to the site docs.

This specific page:

  https://skia.org/dev/testing/skiaperf?cl=1462583002

BUG=skia:
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=1462583002

Review URL: https://codereview.chromium.org/1462583002

site/dev/testing/skiaperf.md

@@ -34,3 +34,18 @@ You can also use the data to answer questions like [how many tests were run per
     count(filter(""))
 
 See Skia Perf for the [full list of functions available](https://perf.skia.org/help).
+
+Embedding
+---------
+
+Once you create a shortcut, which may or may not include calculations, you
+will be presented with the code to embed that graph as an iframe. For example,
+here is an embedding code for showing the ratio of all 565 tests over all 8888
+tests:
+
+    <iframe src='https://perf.skia.org/frame/#4518' width=500 height=300 frameborder=0></iframe>
+
+And the embedded graph appears as:
+
+  <iframe src='https://perf.skia.org/frame/#4518' width=500 height=300 frameborder=0></iframe>
+