mononoke: add README.md and the missing pieces for supporting cargo (#13)

Summary:
Take the README.md from
7ead0e29e4/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

build/fbcode_builder/getdeps/manifest.py

@@ -69,7 +69,10 @@ SCHEMA = {
         },
     },
     "msbuild": {"optional_section": True, "fields": {"project": REQUIRED}},
-    "cargo": {"optional_section": True, "fields": {"build_doc": OPTIONAL}},
+    "cargo": {
+        "optional_section": True,
+        "fields": {"build_doc": OPTIONAL, "workspace_dir": OPTIONAL},
+    },
     "cmake.defines": {"optional_section": True},
     "autoconf.args": {"optional_section": True},
     "b2.args": {"optional_section": True},
@@ -421,6 +424,7 @@ class ManifestParser(object):
 
         if builder == "cargo":
             build_doc = self.get("cargo", "build_doc", False, ctx)
+            workspace_dir = self.get("cargo", "workspace_dir", "", ctx)
             return CargoBuilder(
                 build_options,
                 ctx,
@@ -429,6 +433,7 @@ class ManifestParser(object):
                 build_dir,
                 inst_dir,
                 build_doc,
+                workspace_dir,
                 loader,
             )