Update Codecov instructions
Update Codecov instructions

The current Codecov bash script automatically searches in the DerivedData directory for code coverage files (both gcov and profdata files). In order to prevent the codecov script to automatically search for coverage data and generate reports, we disable report generation (-X coveragepy -X gcov -X xcode) and tell it to upload the report generated by slather (-f path/to/xml_report/cobertura.xml).

SlatherOrg/slather,venmo/slather