Add `README.fuchsia` to third-party directories

[joshlf]

In attempting to roll zerocopy 0.7.10 into Fuchsia, I discovered that our use of vendored third-party code does not play nicely with Fuchsia's automated license hygiene enforcement:

check-licenses: failed to analyze the given directory: All files and folders must have proper license attribution.
This means a license file needs to accompany all first and third party projects,
and a README.fuchsia file must exist and specify where the license file lives.
The following directories are not included in a project:

-> third_party/rust_crates/vendor/zerocopy-0.7.10/src/third_party/rust

Please add a LICENSE file to the above projects, and point to them in an associated README.fuchsia file.
If this is urgent and you cannot complete the above steps, add the above paths to the relevant allowlist
in tools/check-licenses/result/_config.json (last resort).

See go/fuchsia-licenses-playbook for information on resolving common errors.

We should add a README.fuchsia file as suggested so that Fuchsia can vendor zerocopy.