message: Recover docs for git vendoring
  Cherry-picked from which failed
  to update the book and was replaced in
diff: @@ -20,6 +20,12 @@ like so:
  [source.my-awesome-source]
  directory = "vendor"
  +# Git sources can optionally specify a branch/tag/rev as well
  +git = "https://example.com/path/to/repo"
  +# branch = "master"
  +# tag = "v1.0.1"
  +# rev = "313f44e8"
  +
  # The crates.io default source for crates is available under the name
  # "crates-io", and here we use the `replace-with` key to indicate that it's
  # replaced with our source above.
  