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

codecov.yml

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