Add @dotnet/wpf-developers to CODEOWNERS to get automatic inclusion into all PR's (#2888)

CODEOWNERS

@@ -0,0 +1,6 @@
+# Lines starting with '#' are comments.
+# Each line is a file pattern followed by one or more owners.
+
+# These owners will be the default owners for everything in the repo,
+# and will automatically be added as reviewers to all pull requests.
+*       @dotnet/wpf-developers