Update CONTRIBUTING.md

Add a bit on test coverage (intentionally somewhat vague atm) and fix contributor/collaborator mixup.

CONTRIBUTING.md

@@ -10,6 +10,7 @@ Most submissions, including submissions by project collaborators, require review
 - You've read this document, the [Contributing Guidelines](CONTRIBUTING.md).
 - Your changes are consistent with the [Coding Style](https://github.com/tensorflow/lucid/blob/master/CONTRIBUTING.md#coding-style).
 - You've run all [Unit Tests](https://github.com/tensorflow/lucid/blob/master/CONTRIBUTING.md#unit-tests) on all supported versions of Python.
+- You've added at least integration-level unit tests for your code. A reasonable indicator is that your PR doesn't substantially reduce [test coverage](https://coveralls.io/github/tensorflow/lucid). 
 - If you've added new files, you've [included a License](https://github.com/tensorflow/lucid/blob/master/CONTRIBUTING.md#unit-tests) at the top of those files.
 - You've signed Google's [Contributor License Agreement (CLA)](https://cla.developers.google.com/). No worries about this—you do [not surrender ownership of your contribution, and you do not give up any of your rights to use your contribution elsewhere](https://cla.developers.google.com/about).
 
@@ -80,4 +81,4 @@ If you want to be involved more closely, we do welcome all help and are generall
 
 - You have submitted at least one substantial pull request that was merged.
 - You have a reasonable expectation of making additional future contributions of similar substance.
-- The contributor status makes it easier for you to contribute. This could be, for example, because you contribute frequent small patches, or because you need to administrate this Github repository.
+- The collaborator status makes it easier for you to contribute. This could be, for example, because you contribute frequent small patches, or because you need to administrate this Github repository.