[ASTImporter] Add development internals docs

Reviewers: a_sidorin, shafik, teemperor, gamesh411, balazske, dkrupp, a.sidorin

Subscribers: rnkovacs, Szelethus, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66336

llvm-svn: 371839