rename codecov.yml to .codecov.yml

Make it a hidden file. From the [CodeCov docs][1]:

> Can I name the file .codecov.yml?
>
> Yes, you can name the file `codecov.yml` or `.codecov.yml`. However, the
> file must still be located in the repository root, `dev/`, or `.github/`
> directories

[1]: https://docs.codecov.com/docs/codecov-yaml#can-i-name-the-file-codecovyml

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.codecov.yml

@@ -0,0 +1,18 @@
+comment: false
+
+github_checks:
+  annotations: false
+
+coverage:
+  status:
+    patch: false
+    # project will give us the diff in the total code coverage between a commit
+    # and its parent
+    project:
+      default:
+        target: auto
+        threshold: "15%"
+    changes: false
+
+ignore:
+  - "vendor/**/*"