Update changelog

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

CHANGELOG.md

@@ -6,3 +6,10 @@ NC 11 (????-??-??)
 * PHP 5.4 and 5.5 no longer supported
 * PHP 7.1 support
 * OC_L10N removed use \OCP\IL10N (#1948)
+* Preview handling is improved by sharing previews:
+  * Preview sharing (shared files/external storages)
+  * Previews are stored in the AppData
+  * Previews are served faster by not first converting them to image objects
+* Core preview route changed:
+  * Route for the urlgenerator changed from 'core_ajax_preview' to 'core.Preview.getPreview'
+  * $urlGenerator->linkToRoute('core_ajax_preview', ...) => $urlGenerator->linkToRoute('core.Preview.getPreview', ...)