Added myself as editor for CSS Backgrounds 4

Author: SebastianZ

css-backgrounds-4/Overview.bs

@@ -10,6 +10,7 @@ ED: https://drafts.csswg.org/css-backgrounds-4/
 Editor: Bert Bos, W3C, bert@w3.org, w3cid 3343
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Editor: Lea Verou, Invited Expert, http://lea.verou.me/about/, w3cid 52258
+Editor: Sebastian Zartner, Invited Expert, sebastianzartner@gmail.com, w3cid 64937
 Abstract: This module contains the features of CSS relating to the borders and backgrounds of boxes on the page. It includes and extends the functionality of <a href="https://www.w3.org/TR/css3-background/">CSS Backgrounds and Borders Level&nbsp;3</a>. [[CSS3BG]] The main extensions compared to level&nbsp;3 are shaping ('corner-shape') and clipping borders ('border-clip'), logical background positions ('background-position'), and the ''extend'' ability of 'background-repeat'.
 Ignored Terms:
 Warning: Not Ready