Update fosuser-bundle.md
Update fosuser-bundle.md

Add example to create User with the serialization groups.

leesiongchan/DunglasApiBundle,theofidry/DunglasApiBundle,teohhanhui/api-platform-core,vincentchalamon/core,ahlee2326/DunglasApiBundle,soyuka/DunglasApiBundle,jdeniau/DunglasApiBundle,teohhanhui/api-platform-core,meyerbaptiste/core,api-platform/core,jdeniau/DunglasApiBundle,api-platform/core,lucascherifi/DunglasApiBundle,soyuka/core,teohhanhui/api-platform-core,soyuka/core,dunglas/DunglasApiBundle,soyuka/core,jdeniau/DunglasApiBundle,api-platform/core,jdeniau/DunglasApiBundle,vincentchalamon/core,teohhanhui/api-platform-core,yelmontaser/DunglasApiBundle,soyuka/core,api-platform/core,teohhanhui/DunglasApiBundle,meyerbaptiste/core,meyerbaptiste/core,vincentchalamon/core,vincentchalamon/core,meyerbaptiste/core