docs: Specify that we will support prior major versions

This came up in a [reddit
discussion](https://www.reddit.com/r/rust/comments/vbep9q/clap_32_last_call_before_40/ic91vul/)
and seems like a way we can reduce concerns over clap churn without an
undue burden on maintainers.

CONTRIBUTING.md

@@ -60,6 +60,19 @@ Some practices to avoid breaking changes
   - Please keep API addition and deprecation in separate commits in a PR to make it easier to review
 - Develop the feature behind an `unstable-<name>` feature flag with a stablization tracking issue (e.g. [Multicall Tracking issue](https://github.com/clap-rs/clap/issues/2861))
 
+### Version Support Policy
+
+As we work towards [a more flexible architecture](https://github.com/clap-rs/clap/discussions/3476), we hope to support multiple major versions to help ease users through the upgrade churn.
+
+| Version                                              | Status        | Support | End-of-Life |
+|------------------------------------------------------|---------------|---------|
+| [v4](https://github.com/clap-rs/clap/tree/master)    | active        | Features and bug fixes target `master` by default | TBD |
+| [v3](https://github.com/clap-rs/clap/tree/v3-master) | maintenance   | Accepting trivial cherry-picks from `master` (i.e. minimal conflict resolution) by contributors | TBD |
+| [v2](https://github.com/clap-rs/clap/tree/v2-master) | deprecated    | Only accepting fixes for ecosystem-wide critical bugs | TBD |
+| v1                                                   | unsupported   | \- | \- |
+
+Note: We have not yet determined the End-of-Life schedule for previous major versions.  We will give at least a 2 month warning before changing the support status.
+
 ### Testing Code
 
 To test with all features both enabled and disabled, you can run this command: