Add codecov.yml file. (#2847)

* Disable codecov pull request comments
* define range 90% to 100% as 'green'

codecov.yml

@@ -0,0 +1,8 @@
+# Disable pull request comments, they're just
+# spammy and inacurate 99% of the time
+comment: false
+
+coverage:
+    precision: 2
+    round: nearest
+    range: 90...100