adding universal header (dotnet#1750)

Author: mairaw

docfx.json

@@ -79,7 +79,8 @@
       "author": "dotnet-bot",
       "ms.author": "dotnetcontent",
       "manager": "wpickett",
-      "searchScope": [".NET"]
+      "searchScope": [".NET"],
+      "uhfHeaderId": "MSDocsHeader-DotNet"
     },
     "fileMetadata": {
       "ms.prod":
@@ -95,4 +96,4 @@
     "template": [ "docs.html" ],
     "xref": ["_zip/msdn.4.5.2.zip", "_zip/namespaces.4.5.2.zip", "_zip/vs.110.zip", "_zip/vs.140.zip", "_zip/office15.zip", "_zip/missingapi.yml"]
   }
-}
+}