Add Elafros project roles to CONTRIBUTING.md (#224)
Add Elafros project roles to CONTRIBUTING.md (#224)

These roles have been authored by dewitt@ and mchmarny@.

This set of role definitions will also change our development
process such that:

1. A narrower group of individuals will be able to approve PRs
(maintainers)
2. Maintainers will be responsible for merging PRs
knative/serving,knative/serving