Update link to list of maintainers

[matthiasr]

The Google Sheet is deprecated, the canonical link to the maintainers list is http://maintainers.cncf.io/ now.

Signed-off-by: Matthias Rampke matthias@prometheus.io

Please make sure that you follow instructions from CONTRIBUTING

Specially:

• Check if all tests pass, see TESTING for deatils.
• Make sure you've added test coverage for new features/metrics.
• Make sure you have updated documentation.
• If you added a new metric, please make sure you have been following instructions about adding new metric.