docs: document version support goals (#927)

README.md

@@ -137,6 +137,27 @@ has instructions for running the samples.
 The [Google Cloud Pub/Sub Node.js Client API Reference][client-docs] documentation
 also contains samples.
 
+## Supported Node.js Versions
+
+Our client libraries follow the [Node.js release schedule](https://nodejs.org/en/about/releases/).
+Libraries are compatible with all current _active_ and _maintenance_ versions of
+Node.js.
+
+Client libraries targetting some end-of-life versions of Node.js are available, and
+can be installed via npm [dist-tags](https://docs.npmjs.com/cli/dist-tag).
+The dist-tags follow the naming convention `legacy-(version)`.
+
+_Legacy Node.js versions are supported as a best effort:_
+
+* Legacy versions will not be tested in continuous integration.
+* Some security patches may not be able to be backported.
+* Dependencies will not be kept up-to-date, and features will not be backported.
+
+#### Legacy tags available
+
+* `legacy-8`: install client libraries from this dist-tag for versions
+  compatible with Node.js 8.
+
 ## Versioning
 
 This library follows [Semantic Versioning](http://semver.org/).
@@ -160,6 +181,12 @@ More Information: [Google Cloud Platform Launch Stages][launch_stages]
 
 Contributions welcome! See the [Contributing Guide](https://github.com/googleapis/nodejs-pubsub/blob/master/CONTRIBUTING.md).
 
+Please note that this `README.md`, the `samples/README.md`,
+and a variety of configuration files in this repository (including `.nycrc` and `tsconfig.json`)
+are generated from a central template. To edit one of these files, make an edit
+to its template in this
+[directory](https://github.com/googleapis/synthtool/tree/master/synthtool/gcp/templates/node_library).
+
 ## License
 
 Apache Version 2.0