Introduce OpenRefine extensions and alternative distributions

[bencomp]

How could the content be improved?

This is a specific suggestion from #29 to incorporate content about extensions for and/or distributions ("packages") of OpenRefine.

The minutes of the 2018 CAC meeting include:

Add something to talk about packages/extensions and their power.

I commented in #29 (comment):

If I understand correctly, packages in the action items refers to extensions and distributions for OpenRefine. I think that could be a topic for the discussion page/section, because support for extensions across OR versions varies a lot and the distributions appear to be niche products. They can be powerful, but I feel they are less suited to people who first discover OR.
As I did install the RDF extension my screen looks a bit different from the learners' screens, which I do point out at the beginning of the lesson.

@ostephens commented in #29 (comment):

Unless there is something really specific I'd say these are worth mentioning but not including in detail (this is the approach we take in Library Carpentry).

In summary, I think we may:

• Add an Instructor note about possible differences in the layout of the user interface when extensions are installed
• Add a few sentences (to Other Resources) about the existence of extensions and alternative distributions of OpenRefine, with the remark that novice users probably do not need them