message: docs: document version support goals
diff: @@ -74,6 +74,8 @@ async function quickstart(
  // The metadata associated with the entry
  const metadata = {
  resource: {type: 'global'},
  + // See: https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry#logseverity
  + severity: 'INFO',
  };
  // Prepares a log entry
  @@ -126,6 +128,27 @@ has instructions for running the samples.
  The [Stackdriver Logging 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/).
  