Adds a fixed codecov.yml target. (#10)

`grcov` can be pretty noisy in coverage, so the auto targets for this
can lead to a lot of check failures that seem pedantic.

Tested with:

`curl --data-binary @.github/codecov.yml https://codecov.io/validate`

Author: almann

.github/codecov.yml

@@ -0,0 +1,10 @@
+coverage:
+  status:
+    project:
+      default:
+        target: 80%
+        threshold: 5%
+    patch:
+      default:
+        target: 85%
+        threshold: 5%