Add docs for relay data mocking
Add docs for relay data mocking

see https://github.com/artsy/reaction/pull/1854#issuecomment-454100304 for context
xtina-starr/reaction,xtina-starr/reaction,xtina-starr/reaction,xtina-starr/reaction,artsy/reaction-force,artsy/reaction-force,artsy/reaction,artsy/reaction,artsy/reaction