add codecov config

codecov.yml

@@ -0,0 +1,18 @@
+coverage:
+  status:
+    project:
+      default: off
+      ixnet:
+        flags: ixnet
+      rxnet:
+        flags: rxnet
+
+flags:
+  ixnet:
+    paths: Ix.NET/Source/
+  rxnet:
+    paths: Rx.NET/Source/src/
+
+codecov:
+  ci:
+    - dev.azure.com