Bug 1520490 - Add title to GeckoView's CHANGELOG.md. r=fluffyemily

This makes the page's HTML title be:

```
API Changelog | geckoview
```

Differential Revision: https://phabricator.services.mozilla.com/D16705

--HG--
extra : moz-landing-system : lando

mobile/android/geckoview/src/main/java/org/mozilla/geckoview/doc-files/CHANGELOG.md

@@ -1,5 +1,7 @@
 ---
 layout: geckoview
+title: API Changelog
+description: GeckoView API Changelog.
 ---
 
 <h1> GeckoView API Changelog. </h1>