chore(all): add CODEOWNERS (#629)

diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
new file mode 100644
index 0000000..e1cfbb5
--- /dev/null
+++ b/.github/CODEOWNERS
@@ -0,0 +1,2 @@
+# Default owner for all directories not owned by others
+* @googleapis/yoshi-go-admins