docs: Add Maintainer's etiquette

docs/maintainer-etiquette.md

@@ -0,0 +1,91 @@
+# Maintainer's Etiquette
+
+Are you a core maintainer of Terraform? Great! Here's a few notes
+to help you get comfortable when working on the project.
+
+## Expectations
+
+We value the time you spend on the project and as such your maintainer status
+doesn't imply any obligations to do any specific work.
+
+### Your PRs
+
+These apply to all contributors, but maintainers should lead by examples! :wink:
+
+ - for `provider/*` PRs it's useful to attach test results & advise on how to run the relevant tests
+ - for `bug`fixes it's useful to attach repro case, ideally in a form of a test
+
+### PRs/issues from others
+
+ - you're welcomed to triage (attach labels to) other PRs and issues
+   - we generally use 2-label system (= at least 2 labels per issue/PR) where one label is generic and other one API-specific, e.g. `enhancement` & `provider/aws`
+
+## Merging
+
+ - you're free to review PRs from the community or other HC employees and give :+1: / :-1:
+ - if the PR submitter has push privileges (recognizable via `Collaborator`, `Member` or `Owner` badge) - we expect **the submitter** to merge their own PR after receiving a positive review from either HC employee or another maintainer. _Exceptions apply - see below._
+ - we prefer to use the Github's interface or API to do this, just click the green button
+ - squash?
+   - squash when you think the commit history is irrelevant (will not be helpful for any readers in T+6mons)
+ - Add the new PR to the **Changelog** if it may affect the user (almost any PR except test changes and docs updates)
+   - we prefer to use the Github's web interface to modify the Changelog and use `[GH-12345]` to format the PR number. These will be turned into links as part of the release process. Breaking changes should be always documented separately.
+
+## Release process
+
+ - HC employees are responsible for cutting new releases
+ - The employee cutting the release will always notify all maintainers via Slack channel before & after each release
+	so you can avoid merging PRs during the release process.
+
+## Exceptions
+
+Any PR that is significantly changing or even breaking user experience cross-providers should always get at least one :+1: from a HC employee prior to merge.
+
+It is generally advisable to leave PRs labelled as `core` for HC employees to review and merge.
+
+Examples include:
+ - adding/changing/removing a CLI (sub)command or a [flag](https://github.com/hashicorp/terraform/pull/12939)
+ - introduce a new feature like [Environments](https://github.com/hashicorp/terraform/pull/12182) or [Shadow Graph](https://github.com/hashicorp/terraform/pull/9334)
+ - changing config (HCL) like [adding support for lists](https://github.com/hashicorp/terraform/pull/6322)
+ - change of the [build process or test environment](https://github.com/hashicorp/terraform/pull/9355)
+
+## Breaking Changes
+
+ - we always try to avoid breaking changes where possible and/or defer them to the nearest major release
+   - [state migration](https://github.com/hashicorp/terraform/blob/2fe5976aec290f4b53f07534f4cde13f6d877a3f/helper/schema/resource.go#L33-L56) may help you avoid breaking changes, see [example](https://github.com/hashicorp/terraform/blob/351c6bed79abbb40e461d3f7d49fe4cf20bced41/builtin/providers/aws/resource_aws_route53_record_migrate.go)
+   - either way BCs should be clearly documented in special section of the Changelog
+ - Any BC must always receive at least one :+1: from HC employee prior to merge, two :+1:s are advisable
+
+ ### Examples of Breaking Changes
+
+  - https://github.com/hashicorp/terraform/pull/12396
+  - https://github.com/hashicorp/terraform/pull/13872
+  - https://github.com/hashicorp/terraform/pull/13752
+
+## Unsure?
+
+If you're unsure about anything, ask in the committer's Slack channel.
+
+## New Providers
+
+These will require :+1: and some extra effort from HC employee.
+
+We expect all acceptance tests to be as self-sustainable as possible
+to keep the bar for running any acceptance test low for anyone
+outside of HashiCorp or core maintainers team.
+
+We expect any test to run **in parallel** alongside any other test (even the same test).
+To ensure this is possible, we need all tests to avoid sharing namespaces or using static unique names.
+In rare occasions this may require the use of mutexes in the resource code.
+
+### New Remote-API-based provider (e.g. AWS, Google Cloud, PagerDuty, Atlas)
+
+We will need some details about who to contact or where to register for a new account
+and generally we can't merge providers before ensuring we have a way to test them nightly,
+which usually involves setting up a new account and obtaining API credentials.
+
+### Local provider (e.g. MySQL, PostgreSQL, Kubernetes, Vault)
+
+We will need either Terraform configs that will set up the underlying test infrastructure
+(e.g. GKE cluster for Kubernetes) or Dockerfile(s) that will prepare test environment (e.g. MySQL)
+and expose the endpoint for testing.
+