Add example how to enable extension

[hhorak]

TODO: This would deserve a test as well.

[fridex]

Thanks, @hhorak! :+1:

[pkubatrh]

[test][test-openshift]

[mscherer]

Wouldn't it be better for users to have environment variables for that ? For example, if a user want to enable hstore, shipped by default, having to do a separate container is a bit annoying, while it could be done with a variable POSTGRES_EXTENSIONS, who would run CREATE IF NOT EXISTS at start on all extensions in the list (either space separated, or comma separated, whatever is the most consistent with others containers).

[mscherer]

So I went ahead with #399 (with tests and docs).

[phracek]

[test]

[pkubatrh]

[test-openshift]

[fila43]

[test]

[phracek]

@hhorak Please generate also source via distgen. Let's get test it and merge it. Generated sources are outdated.

[hhorak]

Rebased.

[hhorak]

[test]