[ckan-dev] [#2477] Add extensions section to docs

[Sean Hammond]

> This all sounds good to me. When are people planning to go through the
> extensions? I can help and give my perspective about where the various
> functionality fits in in terms of product lines.

Lets just do it my email. So we have the core extensions, looking in
ckan/ckanext it looks like:

- multilingual
- organizations
- publisher_form
- stats

What external extensions do we want to officially support? Some guesses
from me:

- archiver?
- datastorer?
- social?
- harvest?
- inspire?
- disqus?
- qa?
- spatial?

Reply with +1's and -1's for these and other extensions and we'll
compile a list