docs: Add buster and stretch to the image variant list

Author: nschonni

README.md

@@ -23,6 +23,8 @@ The official Node.js docker image, made with love by the node community.
 - [Image Variants](#image-variants)
   - [`node:<version>`](#nodeversion)
   - [`node:alpine`](#nodealpine)
+  - [`node:buster`](#nodebuster)
+  - [`node:stretch`](#nodestretch)
   - [`node:slim`](#nodeslim)
 - [License](#license)
 - [Supported Docker versions](#supported-docker-versions)
@@ -190,6 +192,16 @@ image as a base, add the things you need in your own Dockerfile
 (see the [`alpine` image description](https://hub.docker.com/_/alpine/) for
 examples of how to install packages if you are unfamiliar).
 
+### `node:buster`
+This image is based on version 10 of
+[Debian](http://debian.org), available in
+[the `debian` official image](https://hub.docker.com/_/debian).
+
+### `node:stretch`
+This image is based on version 9 of
+[Debian](http://debian.org), available in
+[the `debian` official image](https://hub.docker.com/_/debian).
+
 ### `node:slim`
 
 This image does not contain the common packages contained in the default tag and