Update upload.md
Update upload.md

added a section under section 4 :

4.1. Implementing functions

a short talk about implementing FileInterface functions and an example.

Restless-ET/AdmingeneratorGeneratorBundle,symfony2admingenerator/AdmingeneratorGeneratorBundle,Restless-ET/AdmingeneratorGeneratorBundle,Restless-ET/AdmingeneratorGeneratorBundle,symfony2admingenerator/AdmingeneratorGeneratorBundle