Add remove instructions to postgres

[hershaw]

Why the changes: When you install the postgres via the marketplace, there's no way to remove it. The feeling of installing something and having no way to remove it is not a very good one.

If your pull request concerns an existing Marketplace application, please make sure you have:

• Notified the Maintainer of the application in this pull request so that they are aware of your proposal. Who is the maintainer and how do I notify them in this pull request?
• Outlined the changes you are proposing, and the reasons these are required (e.g. stability, compatibility with new versions of Kubernetes implementations, etc).
• Tested that the application works with the proposed updates applied (including a screenshot).
• Updated the version number of the app if that has changed. Not sure if this is necessary