jekyll-import warning note of missing deps.

The jekyll-import gem, opted to leave out importer-specific runtime dependencies. I'm adding a warning note for the user to be aware of this and manually install the ones they need based on the importer they are using.
Fix typos.
Show the complete name and no abbreviation.
requieres.
Fix typos.
Merge pull request #1626 from albertogg/migrations-warning-note

jekyll-import warning note of missing deps.