Add a CODEOWNERS file (#6686)

.github/CODEOWNERS

@@ -0,0 +1,11 @@
+# Users referenced in this file will automatically be requested as reviewers for PRs that modify the given paths.
+# See https://help.github.com/articles/about-code-owners/
+
+/build/ @natemcmaster
+/eng/ @natemcmaster
+/src/DefaultBuilder/ @tratcher
+/src/Hosting/ @tratcher
+/src/Http/ @tratcher
+/src/Middleware/ @tratcher
+/src/Security/ @tratcher
+/src/Servers/ @tratcher