Summary:
* Adds a `README.md` file.
* Adds optional features to enable memory access and the async API.
`safeptrace` depends on `reverie-process` and `reverie-memory`, so those will need to be published as crates too.
Reviewed By: VladimirMakaev
Differential Revision: D44386559
fbshipit-source-id: e74c55d5d26239aa09aedac130dca880eb5f1206