Put Codecov in informational mode

Automatically rejecting PRs on dropping coverage doesn't work well
in practice, because there are situations in which coverage drops
while code improves. For example removing code that had test coverage
but is no longer needed will lower total coverage, while removing
dead code can lead to low diff coverage.

With this change, it is up to reviewers to check whether a dropping
coverage is a cause for concern (missing test cases for new/changed
functionality) or expected.

.codecov.yml

@@ -0,0 +1,8 @@
+coverage:
+  status:
+    project:
+      default:
+        informational: true
+    patch:
+      default:
+        informational: true