[ci] Add .codecov.yml

Similar to https://github.com/GoogleChrome/lighthouse/blob/master/.codecov.yml

No strong preference on commenting, feel free to enable it again
diff --git a/.codecov.yml b/.codecov.yml
new file mode 100644
index 0000000..e9b8ab4
--- /dev/null
+++ b/.codecov.yml
@@ -0,0 +1,7 @@
+comment: off
+
+coverage:
+  status:
+    project:
+      default:
+        threshold: 1%