Clarify release branch cherry-picks process in CONTRIBUTING.md (#8634)

### Motivation:

SwiftPM's contributor documentation should clarify that in general changes should be submitted against the `main` branch first, with organization-wide PR description templates to be followed for release branch cherry-picks.

### Modifications:

Added a paragraph to `CONTRIBUTING.md` clarifying the process of the `swiftlang` GitHub org for release branches.

### Result:

Contributors documentation is more clear and it's more obvious how a change could be cherry-picked to release branches.

Author: MaxDesiatov

CONTRIBUTING.md

@@ -343,6 +343,8 @@ your contribution to Apple and the community, and agree by submitting the patch
 that your contributions are licensed under the [Swift
 license](https://swift.org/LICENSE.txt).
 
+After a change is known not to cause regressions in the `main` branch, it may be considered for cherry-picking to the latest release branch depending on the release schedule. Cherry-picks require [a specific template to be followed](https://github.com/swiftlang/.github/blob/main/PULL_REQUEST_TEMPLATE/release.md) in PR description that consolidates information about the change necessary for inclusion in the release branch and provides risk evaluation for nominating the change.
+
 ## Continuous Integration
 
 SwiftPM uses [swift-ci](https://ci.swift.org) infrastructure for its continuous integration testing. The bots can be triggered on pull-requests if you have commit access. Otherwise, ask one of the code owners to trigger them for you.