Add codeowners (#13)

Author: nkolev92

.github/CODEOWNERS

@@ -0,0 +1,5 @@
+# These owners will be the default owners for everything in
+# the repo. Unless a later match takes precedence,
+# review when someone opens a pull request.
+# For more on how to customize the CODEOWNERS file - https://help.github.com/en/articles/about-code-owners
+*       @NuGet/nuget-client