Summary:
Take the README.md from
https://github.com/facebookexperimental/mononoke/blob/7ead0e29e41aec0771531a4650b6170bc1ff6566/README.md
and apply it on Eden repo.

Re-add the Cargo.toml file that declares Cargo workspace.

Re-add fbcode_builder/getdeps manifest for Mononoke
Pull Request resolved: https://github.com/facebookexperimental/eden/pull/13

Test Plan:
./build/fbcode_builder/getdeps.py build mononoke
  ./build/fbcode_builder/getdeps.py test mononoke

Reviewed By: ahornby

Differential Revision: D19833059

Pulled By: lukaspiatkowski

fbshipit-source-id: fb37e13306c0b9969a7c4e52b05e1a66a577022f