users-guide: Add documentation for JSON profile format
users-guide: Add documentation for JSON profile format

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3184

ezyang/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc