Add README.md for Vztore vector database

[Copilot]

Documents Vztore as a key-value based vector database, providing usage examples and API reference based on src/main.zig.

Documentation includes:

• Overview of dual-storage architecture (NMSLIB for vector indexing, LMDBX for persistence)
• Features: sparse vectors, HNSW-based ANN search, batch operations, auto-generated UUIDs
• Installation via Zig package manager
• API reference for VStore methods (init, batchPut, batchGet, search, save, deinit)

Example usage:

var store = try VStore.init(
    allocator,
    "my_database",
    "negdotprod_sparse",
    nmslib.DataType.SparseVector,
    nmslib.DistType.Float,
    1000,
);
defer store.deinit() catch unreachable;

const keys = try store.batchPut(&vectors, &data, null);
const result = try store.batchGet(keys[0]);

Original prompt

Read src/main.zig. Generate README.md. Vztore is key-value based vector database

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