Update Blueprints.md with info on overriding controller blueprints

Added information on how to override blueprints in a Sails project, taken from https://stackoverflow.com/questions/22273789/crud-blueprint-overriding-in-sailsjs and relevant to #317 and #835.

Also explaining the catch with lowercasing files in /api/blueprints, but maybe this is a bug?
Happens in lib/hooks/blueprints/index.js -> loadMiddleware

      // Merge custom overrides from our app into the BlueprintController
      // in Sails core.
      _.extend(BlueprintController, modules);

BlueprintController contains everything lowercased and modules is camelcased. (But modules already contain a lowercased "identity" property – maybe that can be used for merging?)

Cheers!
M