docs: add readme to cw-contract crate

ibc-clients/ics07-tendermint/cw-contract/README.md

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+# `ibc-client-tendermint-cw` crate
+
+This crate showcases how to reuse an `ibc-rs` light client as a
+[CosmWasm contract](https://github.com/cosmos/ibc/blob/main/spec/client/ics-008-wasm-client/README.md)
+utilizing the `ibc-client-cw` crate.
+
+The `ibc-client-cw` crate exposes the requisite types and traits needed to reuse
+the `ibc-rs` light clients. Notably, it offers a
+[`ClientType`](https://docs.rs/ibc-client-cw/latest/ibc_client_cw/api/trait.ClientType.html)
+trait, which requires two associated types: `ClientState` and `ConsensusState`.
+These types take any type that implement the
+[`ClientStateExecution`](https://docs.rs/ibc-core/latest/ibc_core/client/context/client_state/trait.ClientStateExecution.html)
+and
+[`ConsensusState`](https://docs.rs/ibc-core/latest/ibc_core/client/context/consensus_state/trait.ConsensusState.html)
+traits from the `ibc-core` crate.
+
+For example, to reuse the existing
+[`ibc-client-tendermint`](https://docs.rs/ibc-client-tendermint/latest/ibc_client_tendermint/):
+
+```rs
+use ibc_client_cw::api::ClientType;
+use ibc_client_tendermint::client_state::ClientState;
+use ibc_client_tendermint::consensus_state::ConsensusState;
+
+#[derive(Clone, Debug)]
+pub struct TendermintClient;
+
+impl<'a> ClientType<'a> for TendermintClient {
+    type ClientState = ClientState;
+    type ConsensusState = ConsensusState;
+}
+```
+
+Once the `ClientType` trait is implemented, the `ibc-client-cw` crate can be
+used to complete the entry points for the CosmWasm contract:
+
+```rs
+use cosmwasm_std::{entry_point, Binary, Deps, DepsMut, Env, MessageInfo, Response};
+use ibc_client_cw::context::Context;
+use ibc_client_cw::types::{ContractError, InstantiateMsg, QueryMsg, SudoMsg};
+
+pub type TendermintContext<'a> = Context<'a, TendermintClient>;
+
+#[entry_point]
+pub fn instantiate(
+    deps: DepsMut<'_>,
+    env: Env,
+    _info: MessageInfo,
+    msg: InstantiateMsg,
+) -> Result<Response, ContractError> {
+    let mut ctx = TendermintContext::new_mut(deps, env)?;
+    let data = ctx.instantiate(msg)?;
+    Ok(Response::default().set_data(data))
+}
+
+#[entry_point]
+pub fn sudo(deps: DepsMut<'_>, env: Env, msg: SudoMsg) -> Result<Response, ContractError> {
+    let mut ctx = TendermintContext::new_mut(deps, env)?;
+    let data = ctx.sudo(msg)?;
+    Ok(Response::default().set_data(data))
+}
+
+#[entry_point]
+pub fn query(deps: Deps<'_>, env: Env, msg: QueryMsg) -> Result<Binary, ContractError> {
+    let ctx = TendermintContext::new_ref(deps, env)?;
+    ctx.query(msg)
+}
+```
+
+The above snippets compile into a fully working CosmWasm contract that implements the
+Tendermint IBC light client.