docs(CONTRIBUTING): add Helm 2's support contract

Signed-off-by: Matthew Fisher <matt.fisher@microsoft.com>

CONTRIBUTING.md

@@ -111,7 +111,7 @@ A milestone (and hence release) is considered done when all outstanding issues/P
 
 ## Semantic Versioning
 
-Helm maintains a strong commitment to backward compatibility. All of our changes to protocols and formats are backward compatible from Helm 3.0 until Helm 4.0. No features, flags, or commands are removed or substantially modified (other than bug fixes).
+Helm maintains a strong commitment to backward compatibility. All of our changes to protocols and formats are backward compatible from one major release to the next. No features, flags, or commands are removed or substantially modified (unless we need to fix a security issue).
 
 We also try very hard to not change publicly accessible Go library definitions inside of the `pkg/` directory of our source code.
 
@@ -123,6 +123,16 @@ For a quick summary of our backward compatibility guidelines for releases betwee
 - Chart repository functionality MUST be backward compatible
 - Go libraries inside of `pkg/` SHOULD remain backward compatible, though code inside of `cmd/` and `internal/` may be changed from release to release without notice.
 
+## Support Contract for Helm 2
+
+With Helm 2's current release schedule, we want to take into account any migration issues for users due to the upcoming holiday shopping season and tax season. We also want to clarify what actions may occur after the support contract ends for Helm 2, so that users will not be surprised or caught off guard.
+
+After Helm 2.15.0 is released, Helm 2 will go into "maintenance mode". We will continue to accept bug fixes and fix any security issues that arise, but no new features will be accepted for Helm 2. All feature development will be moved over to Helm 3.
+
+6 months after Helm 3.0.0's public release, Helm 2 will stop accepting bug fixes. Only security issues will be accepted.
+
+12 months after Helm 3.0.0's public release, support for Helm 2 will formally end. Download links for the Helm 2 client through Google Cloud Storage, the Docker image for Tiller stored in Google Container Registry, and the Google Cloud buckets for the stable and incubator chart repositories may no longer work at any point. Client downloads through `get.helm.sh` will continue to work, and we will distribute a Tiller image that will be made available at an alternative location which can be updated with `helm init --tiller-image`.
+
 ## Issues
 
 Issues are used as the primary method for tracking anything to do with the Helm project.