Two additions to the Joins documentation in README.md for clarity and correctness. 

First, add the 'integer :id' field directive that's required to the PhotoContainer example, and second specify that you need a version of sunspot greater than 2.1.1 (that is, currently, HEAD).  Without adding the integer :id, the example code will not work (complains about not finding :id field), and  without more recent commits than v2.1.1, the join functionality will not work (complains about :target).