Add a pointer to the relevant paper for InScopeSet
Add a pointer to the relevant paper for InScopeSet

This was non-obvious to me when reading the sources and the
paper provides the motivation and explores the design space.

Test Plan: just a comment

Reviewers: simonpj, austin, ezyang, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1749

(cherry picked from commit 47ccf4d2a0df33f26e66f340e89ca516b4597ed0)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc