docs(node): support "stable"/"preview" release level (#1312) (#1041)

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest
-  digest: sha256:111973c0da7608bf1e60d070e5449d48826c385a6b92a56cb9203f1725d33c3d
+  digest: sha256:5ed10ba99cd1ea8c3a0f29b4c53e8a2723a101952705baed6b61783111c64c1c

README.md

@@ -97,6 +97,7 @@ This library follows [Semantic Versioning](http://semver.org/).
 
 
 
+
 This library is considered to be in **beta**. This means it is expected to be
 mostly stable while we work toward a general availability release; however,
 complete stability is not guaranteed. We will address issues and requests
@@ -105,6 +106,7 @@ against beta libraries with a high priority.
 
 
 
+
 More Information: [Google Cloud Platform Launch Stages][launch_stages]
 
 [launch_stages]: https://cloud.google.com/terms/launch-stages