Referencing the documentation for jekyll-import installation

Currently, there are only beta releases of the jekyll-import gem, so to
install it, you have to do `gem install jekyll-import --pre`. Instead
of giving instructions that don't work right now, reference the
documentation. That way, this doesn't need to be updated again when
jekyll-import has a non-beta version; just the documentation will need
updating.
Merge pull request #1662 from carols10cents/fix-import-error-message

Add `--pre` to the recommended command to install jekyll-import.