feat(userguide): spring process applications & EL

- add section on spring process applications
  #CAM-446

- add note about expression resolving with the shared process engine
  #CAM-444

- add note and example for bootstrapping a managed process engine using
  spring
  #CAM-445
Merge pull request #1 from camunda/spring

feat(userguide): spring process applications & EL

* add section on spring process applications
#CAM-446
* add note about expression resolving with the shared process engine
#CAM-444
* add note and example for bootstrapping a managed process engine using
spring
#CAM-445