Update README: export content to distribution

[ksuess]

Im digging into distributions and thought demo.plone.org would be a good starting point as it is in production.

I found out that the mentioned export view does not exist (anymore?).
I included the call of the plone.exportimport script in the Makefiles.

[stevepiercy]

LGTM, but let's get @ericof, @fredvd, or another person more familiar with this repo to review, too. I have only a vague understanding.

[fredvd]

LGTM, the view has indeed been replaced with a command line script jn the latest plone.distribution versions.
Thank you @ksuess

[davisagli]

@ksuess Thanks!