There's no documented way to bootstrap Cargo.lock, Cargo.Bazel.lock

Following the documentation for [crate_universe](http://bazelbuild.github.io/rules_rust/crate_universe.html), you will get build errors similar to this one:

```
ERROR: Error computing the main repository mapping: no such package '@crate_index//': Not a regular file: /Users/adam/code/santa/Cargo.lock
```

And the same error for Cargo.Bazel.lock.

As far as I can tell:

1. If you're using a Cargo.toml, you bootstrap Cargo.lock using `cargo`. If you're not using Cargo.toml you there is no clear way to bootstrap it.
2. If you're specifying Cargo.Bazel.lock, then the docs don't tell you how to bootstrap it. `CARGO_BAZEL_REPIN=1 bazel ...` will not do it. By experimenting, I've figured out that if you `touch Cargo.Bazel.lock` before running bazel, it'll work.

I would suggest the documentation page should mention:

* How to bootstrap `cargo_lockfile` without a Cargo.toml (I don't know, actually)
* That `lockfile` will be bootstrapped with `touch LOCKFILE && CARGO_BAZEL_REPIN=1 bazel test //`
* Add comments to the first example ("Cargo Workspaces") next to the lines specifying the lockfiles, telling the reader to bootstrap those files.