Add README.
Initial.
Add 'interposition/' from commit '701648864417ee066f03d8f39922953515ed99c2'

git-subtree-dir: interposition
git-subtree-mainline: fca13c11dcede65979edac071d403923bc145717
git-subtree-split: 701648864417ee066f03d8f39922953515ed99c2
Merge commit 'c5e646cbff4f565d6743fb1f581ffcaf43b45da7'
Flush.