Document DDMin install instructions

[tartakovsky]

Summary

• add cargo add and manual dependency instructions to the fixture3-ddmin README
• add root README section for the standalone DDMin library
• bump fixture3-ddmin to 0.1.1 so crates.io receives the README update

Verification

• scripts/verify-all.sh
• scripts/verify-ddmin.sh
• python3 scripts/verify-rust-workspaces.py --g3rs
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --manifest-path packages/ddmin/Cargo.toml -p fixture3-ddmin
• cargo publish --dry-run --allow-dirty --manifest-path packages/ddmin/crates/ddmin/Cargo.toml