(#221) add a simple codecov.yml

This should set the target coverage to 80% and allow
each single PR to let coverage drop by 3%.

While this does not sound ideal, the current (default)
configuration feels much too fragile, and breaks on too
many PRs whose coverage is fine.

Author: nils-a

.codecov.yml

@@ -0,0 +1,17 @@
+codecov:
+  notify:
+    require_ci_to_pass: no
+
+coverage:
+  precision: 2
+  round: down
+  range: "75...100"
+  status:
+    project:
+      default:
+        target: 80%
+        threshold: 3%
+    patch:
+      default:
+        target: 80%
+        threshold: 3%