Add config for codecov

This could also go into .codecov.yml, or be configured through their UI.

Ref: https://docs.codecov.io/docs/codecov-yaml
Ref: https://docs.codecov.io/docs/coverage-configuration
Ref: https://docs.codecov.io/docs/pull-request-comments

Author: blueyed

codecov.yml

@@ -0,0 +1,10 @@
+coverage:
+  status:
+    project: yes
+    patch: yes
+    changes: yes
+
+comment:
+  layout: "header, diff"
+  behavior: default
+  require_changes: no