message: doc: add link to arxiv paper
diff: @@ -5,6 +5,10 @@ Mesh is a drop in replacement for
  [malloc(3)](http://man7.org/linux/man-pages/man3/malloc.3.html) that
  compacts the heap without rewriting application pointers.
  +Mesh is described in an [academic paper (arxiv
  +PDF)](https://arxiv.org/abs/1902.04738) that will appear at PLDI
  +2019.
  +
  Mesh runs on Linux; macOS support should be considered alpha-quality,
  and Windows is a work-in-progress.
  