Add Selectors Level 4 :nth-child() to Most Wanted features for Edge

h/t @bardiharborow for opening https://wpdev.uservoice.com/forums/257854-microsoft-edge-developer/suggestions/15944476
Refs twbs#20143
[skip sauce]

docs/_data/browser-features.yml

@@ -38,6 +38,16 @@
   origin: >
     Bootstrap#20618
 
+-
+  browser: >
+    Microsoft Edge
+  summary: >
+    Implement the [`of <selector-list>` clause](http://caniuse.com/#feat=css-nth-child-of) of the `:nth-child()` pseudo-class
+  upstream_bug: >
+    UserVoice#15944476
+  origin: >
+    Bootstrap#20143
+
 -
   browser: >
     Firefox