Add Interaction in definition/interface.md
Add Interaction in definition/interface.md

Summary: Add interaction.

Reviewed By: AkramaMirza

Differential Revision: D38014306

fbshipit-source-id: 0a2348f9fca604aad67271270b8c8797ea45f803

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift