Document chair election schedule

[lizrice]

The SIG chairs are listed on the README; I think it would be a good idea to also document the dates of their term. (The duration is covered by the standard SIG operating process here https://github.com/cncf/toc/blob/master/sigs/cncf-sigs.md#elections)

[dutchshark]

For @chira001: Next week during our chat let's check if we can also address this to afterwards close this one with PR#65 has been merged this will be the contents of the README in the root of the repository to match the terms.