Readme: update maintenance section (#5)

Author: ohmyfelix

README.md

@@ -28,3 +28,6 @@ Ready-to-use images for NodeJS.
 | NodeJS 8        | Alpine v3.7  | v8            |
 | NodeJS 7        | Alpine v3.6  | v7            |
 | NodeJS 6        | Alpine v3.6  | v6            |
+
+## Maintenance
+See [how to contribute](https://github.com/dockette/.github/blob/master/CONTRIBUTING.md) to this package. Consider to [support](https://github.com/sponsors/f3l1x) **f3l1x**. Thank you for using this package.