docs: update readme

[EthanYuan]

Show more information about Cell's structure in the README, such as args and data.

Reference

//
// Cells Data
//

struct SpvInfo {
    tip_client_id: byte,
}

struct TargetAdjustInfo {
    // The `time` of the latest 2016-th header.
    start_time: Uint32,
    // The `nbits` of the next header.
    next_compact_target: Uint32,
}

struct SpvClient {
    id: byte,
    tip_block_hash: Hash,
    headers_mmr_root: HeaderDigest,
    target_adjust_info: TargetAdjustInfo,
}

//
// Script Args
//

struct SpvTypeArgs {
    type_id: Hash,
    clients_count: byte,
    flags: byte,
}

See https://github.com/ckb-cell/ckb-bitcoin-spv/blob/a13477bd12840fc2cd3e606ceea18d52f9c78e9a/verifier/schemas/types.mol#L29C1-L59C2