[Doc] Add versioning_policy doc (#62)

### What this PR does / why we need it?

This patch add the versioning policy doc for vllm-ascend

Reference:
- https://spark.apache.org/versioning-policy.html
- https://docs.openstack.org/project-team-guide/stable-branches.html
- https://github.com/pytorch/pytorch/blob/main/RELEASE.md

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
preview: https://vllm-ascend--62.org.readthedocs.build/en/62/

Signed-off-by: Yikun Jiang <yikunkero@gmail.com>

README.md

@@ -10,7 +10,7 @@ vLLM Ascend Plugin
 </h3>
 
 <p align="center">
-| <a href="https://www.hiascend.com/en/"><b>About Ascend</b></a> | <a href="https://slack.vllm.ai"><b>Developer Slack (#sig-ascend)</b></a> |
+| <a href="https://www.hiascend.com/en/"><b>About Ascend</b></a> | <a href="https://vllm-ascend.readthedocs.io/en/latest/"><b>Documentation</b></a> | <a href="https://slack.vllm.ai"><b>Developer Slack (#sig-ascend)</b></a> |
 </p>
 
 <p align="center">
@@ -77,6 +77,15 @@ We welcome and value any contributions and collaborations:
 - Please feel free comments [here](https://github.com/vllm-project/vllm-ascend/issues/19) about your usage of vLLM Ascend Plugin.
 - Please let us know if you encounter a bug by [filing an issue](https://github.com/vllm-project/vllm-ascend/issues).
 
+## Branch
+
+vllm-ascend has main branch and dev branch.
+
+- **main**: main branch，corresponds to the vLLM main branch, and is continuously monitored for quality through Ascend CI.
+- **vX.Y.Z-dev**: development branch, created with part of new releases of vLLM. For example, `v0.7.1-dev` is the dev branch for vLLM `v0.7.1` version.
+
+Please refer to [Versioning policy](docs/source/developer_guide/versioning_policy.md) for more details.
+
 ## License
 
 Apache License 2.0, as found in the [LICENSE](./LICENSE) file.