chore: checkpoint before Python removal

vendor/zerotrie/README.md

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+# zerotrie [![crates.io](https://img.shields.io/crates/v/zerotrie)](https://crates.io/crates/zerotrie)
+
+<!-- cargo-rdme start -->
+
+A data structure offering zero-copy storage and retrieval of byte strings, with a focus
+on the efficient storage of ASCII strings. Strings are mapped to `usize` values.
+
+[`ZeroTrie`] does not support mutation because doing so would require recomputing the entire
+data structure. Instead, it supports conversion to and from [`LiteMap`] and [`BTreeMap`].
+
+There are multiple variants of [`ZeroTrie`] optimized for different use cases.
+
+## Examples
+
+```rust
+use zerotrie::ZeroTrie;
+
+let data: &[(&str, usize)] = &[("abc", 11), ("xyz", 22), ("axyb", 33)];
+
+let trie: ZeroTrie<Vec<u8>> = data.iter().copied().collect();
+
+assert_eq!(trie.get("axyb"), Some(33));
+assert_eq!(trie.byte_len(), 18);
+```
+
+## Internal Structure
+
+To read about the internal structure of [`ZeroTrie`], build the docs with private modules:
+
+```bash
+cargo doc --document-private-items --all-features --no-deps --open
+```
+
+[`LiteMap`]: litemap::LiteMap
+[`BTreeMap`]: alloc::collections::BTreeMap
+
+<!-- cargo-rdme end -->
+
+## More Information
+
+For more information on development, authorship, contributing etc. please visit [`ICU4X home page`](https://github.com/unicode-org/icu4x).