Add a note about interaction of the new and existing build systems.
Add a note about interaction of the new and existing build systems.

[skip ci]
snowleopard/shaking-up-ghc,sdiehl/ghc,izgzhen/hadrian,sdiehl/ghc,sdiehl/ghc,bgamari/shaking-up-ghc,snowleopard/hadrian,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc