Add preload+joins alternative option for overcoming the known issue with includes

In face of this known issue, .joins(...).preload(...) felt like a slightly better option (vs. :select option passed to .near()) for just the particular use case I'm up against where includes would have been useful. 

Thought it'd be worth adding to the docs, especially as preload() isn't widely demonstrated in Rails own docs.
Merge pull request #488 from eliotsykes/patch-3

Describe preload+joins option for overcoming includes issue.