PROCESS: Instruct gradual rollout for feature flags

We are at a scale where an error in Gitaly can lead to a site-wide outage. In order to minimize this, every feature flag rollout should be done gradually. Encourage this through the feature flag issue template.

When we roll out feature flags, we always want to roll out gradually rather than all at once. Additionally, when turning on feature flags, we should use certain Slack channels for maximum visibility. Explicitly include these instructions in the PROCESS.md doc.