ghci users guide: mention "~" expansion in :add
ghci users guide: mention "~" expansion in :add

I discovered this feature via Stack Overflow.
https://stackoverflow.com/questions/42310399

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3150

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