[FEATURE] Update feature branch guidance for how to update branches

[joshuarrrr]

Is your feature request related to a problem?

We've seen some confusion when it comes to how to keep feature branches updated and sync'd with main. We can provide some additional guidance in https://github.com/opensearch-project/.github/blob/main/CONTRIBUTING.md#feature-branches to reduce that.

What solution would you like?

The feature team is responsible for opening a PR when they would like the feature branch to be updated. The simple and easiest way is to just raise a PR from main to the feature branch.

What alternatives have you considered?

Do you have any additional context?

An example of a PR with confusion: opensearch-project/OpenSearch-Dashboards#3273

[dblock]

Can we introduce automation for this? Maybe a bot that merges main to feature branches on demand that's not a PR?

[joshuarrrr]

We could, but that removes some control from a feature team, and requires more frequent pulls. But it may be a tradeoff worth making.

[dblock]

Maybe automation could be making these PRs all the time? The problem is always that conflicts need resolving, and at some point a human has to do it.