Add reference link to mention of "N+1 Query Problem"

[chaotic3quilibrium]

In README.md, please add the following reference link to the mentioning of the "N+1 Query Problem". It's for those of us who haven't seen this particular problem described using this vernacular before: https://stackoverflow.com/questions/97197/what-is-the-n1-selects-problem-in-orm-object-relational-mapping

[AugustNagro]

Done. Thanks for the link.