Add CODEOWNERS file with default owners (#833)

Set default code owners for the repository.

Author: adityapatwardhan

.github/CODEOWNERS

@@ -0,0 +1,7 @@
+# https://help.github.com/articles/about-codeowners/
+
+# These owners will be the default owners for everything in
+# the repo. Unless a later match takes precedence,
+# the following owners will be requested for
+# review when someone opens a pull request.
+*       @adityapatwardhan @sdwheeler