Add links to v0.15.1 docs

We tag each release so that future generations can see how Pipelines'
documentation looked at a given point in time.

This commit adds a reference to the 0.15.1 documentation from our
README.

README.md

@@ -38,6 +38,7 @@ a cluster with **Kubernetes version 1.16 or later***.
 | Version | Docs | Examples |
 | ------- | ---- | -------- |
 | [HEAD](DEVELOPMENT.md#install-pipeline) | [Docs @ HEAD](/docs/README.md) | [Examples @ HEAD](/examples) |
+| [v0.15.1](https://github.com/tektoncd/pipeline/releases/tag/v0.15.1) | [Docs @ v0.15.1](https://github.com/tektoncd/pipeline/tree/v0.15.1/docs#tekton-pipelines) | [Examples @ v0.15.1](https://github.com/tektoncd/pipeline/tree/v0.15.1/examples#examples) |
 | [v0.15.0](https://github.com/tektoncd/pipeline/releases/tag/v0.15.0) | [Docs @ v0.15.0](https://github.com/tektoncd/pipeline/tree/v0.15.0/docs#tekton-pipelines) | [Examples @ v0.15.0](https://github.com/tektoncd/pipeline/tree/v0.15.0/examples#examples) |
 | [v0.14.3](https://github.com/tektoncd/pipeline/releases/tag/v0.14.3) | [Docs @ v0.14.3](https://github.com/tektoncd/pipeline/tree/v0.14.3/docs#tekton-pipelines) | [Examples @ v0.14.3](https://github.com/tektoncd/pipeline/tree/v0.14.3/examples#examples) |
 | [v0.14.2](https://github.com/tektoncd/pipeline/releases/tag/v0.14.2) | [Docs @ v0.14.2](https://github.com/tektoncd/pipeline/tree/v0.14.2/docs#tekton-pipelines) | [Examples @ v0.14.2](https://github.com/tektoncd/pipeline/tree/v0.14.2/examples#examples) |