point to relevant issues on code coverage and testing

Author: jhamman

docs/developers/contributing.rst

@@ -147,6 +147,7 @@ also collected automatically via the Codecov service.
 .. note::
     Previous versions of Zarr-Python made extensive use of doctests. These tests were
     not maintained during the 3.0 refactor but may be brought back in the future.
+    See :issue:`2614` for more details.
 
 Code standards - using pre-commit
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