docs: add readme pointer to versioning documentation (#12819)

This change puts the version-maintenance policy in a more discoverable
location to reduce confusion about what versions of ddtrace are
maintained.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [ ] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: emmettbutler

README.md

@@ -21,6 +21,9 @@ For advanced usage and configuration information, check out the [library documen
 
 To get started as a contributor, see [the contributing docs](https://ddtrace.readthedocs.io/en/stable/contributing.html) first.
 
+For information about the bug/security fix and maintenance policy, see the [versioning docs][versioning docs].
+
 [setup docs]: https://docs.datadoghq.com/tracing/setup/python/
 [api docs]: https://ddtrace.readthedocs.io/
 [visualization docs]: https://docs.datadoghq.com/tracing/visualization/
+[versioning docs]: https://github.com/DataDog/dd-trace-py/blob/main/docs/versioning.rst#release-support