users-guide: Produce OpenSearch description
users-guide: Produce OpenSearch description

Reviewers: austin, hsyl20

Reviewed By: hsyl20

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2921

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