Add @njeffrie to global codeowners (#74)

keep @advaitjain as the sole codeowners for .github and ci
diff --git a/CODEOWNERS b/CODEOWNERS
index e27f86f..659a61f 100644
--- a/CODEOWNERS
+++ b/CODEOWNERS
@@ -1 +1,4 @@
-*     @advaitjain
+*          @advaitjain @njeffrie
+
+/.github/  @advaitjain
+/ci/       @advaitjain