[crate_universe] Add README to examples/crate_universe directory

[bcmyers]

This adds a README.md to the examples/crate_universe directory in order to explain that the RULES_RUST_CRATE_UNIVERSE_BOOTSTRAP environment variable
must be set to true in order for the examples to "just work."

It also removes reference to bootstrap.sh from the
crate_universe/CONTRIBUTING.md file since bootstrap.sh no longer exists.

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[bcmyers]

@googlebot I signed it!

[illicitonion]

Thanks for the PR (and sorry our docs were out of date so you hit this problem!) - I've made a slight suggestion, as bootstrapping no longer generates a bazelrc file, then very happy to merge!

[bcmyers]

Sounds good to me!

[illicitonion]

Thanks! Would be very interested in any feedback/contributions you may have to crate_universe, as well!