Fix README doctest path so the published crate can be tested

[npuichigo]

Problem

cargo test fails to compile on the crate as published to crates.io:

error: couldn't read `.../rustfst-1.3.1/src/../../README.md`:
       No such file or directory (os error 2)
error: could not compile `rustfst` (lib test)

Reproduce without a checkout:

cargo new /tmp/repro && cd /tmp/repro
cargo add rustfst@1.3.1
cargo vendor >/dev/null && cd vendor/rustfst && cargo test

Cause

rustfst/src/lib.rs has:

#[cfg(test)]
doc_comment!(include_str!("../../README.md"));

../../README.md is correct in the repository — from rustfst/src/ it
resolves to the workspace-root README.

It is wrong in the published package. readme = '../README.md' makes Cargo
copy that file into the package root, so inside the package the README sits next
to src/ and ../../README.md points outside the package.

Normal builds are unaffected because the include is #[cfg(test)], but anyone
vendoring the crate or running its test suite from the registry copy hits it.

Fix

Add rustfst/README.md as a symlink to the workspace README, point readme at
it, and include it as ../README.md — a path that resolves identically in the
repository and in the package. Cargo follows symlinks when packaging, so the
release still ships a real README at the crate root.

Verification

• cargo package --list yields the same README.md entry as before the change.
• cargo test --lib --no-run compiles in-repo.
• Patching the vendored 1.3.1 package the same way makes its lib test target
  compile (it previously failed at compile time). The remaining tests_openfst
  failures there are unrelated and pre-existing — those tests need the
  rustfst-tests-data directory, which is not part of the published package.

One thing to check

generate_readme_from_rustdoc.sh runs cargo sync-readme -f lib from
rustfst/. Writing through the symlink updates the workspace README as before,
but if cargo-sync-readme replaces the file rather than writing through it, the
symlink would be clobbered and would need restoring. I could not verify that
behaviour here.

If you would rather avoid a symlink, two alternatives:

• keep readme = '../README.md' and move the README doctest into an integration
  test under rustfst/tests/, where the packaged path is stable; or
• drop the doc_comment! include entirely if the README examples are covered
  elsewhere.

Happy to switch to whichever you prefer.