v2: contrib/README: add versioning and deprecation policies for contribs

[darccio]

What does this PR do?

Updates our policy on how we deprecate our contribs when upstream deprecates and there is a CVE.

Motivation

Setting clear expectations on how we manage the contribs when the integrated package are deprecated.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[darccio]

Tests are failing due to the current state of v2-dev branch. We are working in some breaking changes.