Update Quality standard (#1214)

Make clear that we have min coverage percent if package is testable.

CONTRIBUTING.md

@@ -32,7 +32,7 @@ To be on the list, project repositories should adhere to these quality standards
   - Or, for finished projects, issues and pull requests are responded to
 - Stable or progressing toward stable
 - Thoroughly documented (README, godoc comments, etc.)
-- Tests, where practical
+- Tests, where practical. If library/program testable, than coverage should be >= 80% for non-data-related packages and >=90% for data related packages. **Notice**: the tests will be reviewed too. We will check your coverage manually if your package's coverage is just a benchmark results.
 
 
 ## Reporting issues