add link to coverage reports

CONTRIBUTING.md

@@ -159,7 +159,8 @@ You might want to begin by looking around the
 [codebase](https://gitlab.torproject.org/tpo/core/arti/), or getting to
 know our [architecture](./doc/Architecture.md).
 
-More tests would always be great.
+More tests would always be great. You can look at the [coverage reports](https://tpo.pages.torproject.net/core/arti/coverage/)
+to find out what parts need the more love.
 
 Parsing more Tor document types would be neat.