Adding docs for client state interface

docs/ibc/light-clients/client-state.md

@@ -1,3 +1,75 @@
 <!--
 order: 2
--->
+-->
+
+# Implementing the `ClientState` interface
+
+Learn how to implement the [`ClientState`](https://github.com/cosmos/ibc-go/blob/v6.0.0-rc1/modules/core/exported/client.go#L40) interface.
+
+## `ClientType` method
+
+`ClientType` should return a unique string identifier of the light client. This will be used when generating a client identifier.
+The format is created as follows: `ClientType-{N}` where `{N}` is the unique global nonce associated with a specific client.
+
+## `GetLatestHeight` method
+
+`GetLatestHeight` should return the latest block height that the client state represents.
+
+## `Validate` method
+
+`Validate` should validate every client state field and should return an error if any value is invalid. The light client
+implementor is in charge of determining which checks are required. See the [tendermint light client implementation](https://github.com/cosmos/ibc-go/blob/v6.0.0-rc1/modules/light-clients/07-tendermint/types/client_state.go#L101)
+as a reference.
+
+## `Status` method
+
+`Status` must return the status of the client.
+
+- An `Active` status indicates that clients are allowed to process packets.
+- A `Frozen` status indicates that a client is not allowed to be used.
+- An `Expired` status indicates that a client is not allowed to be used.
+- An `Unknown` status indicates that there was an error in determining the status of a client.
+
+All possible Status types can be found [here](https://github.com/cosmos/ibc-go/blob/v6.0.0-rc1/modules/core/exported/client.go#L26-L36).
+
+This field is returned by the gRPC [QueryClientStatusResponse](https://github.com/cosmos/ibc-go/blob/v6.0.0-rc1/modules/core/02-client/types/query.pb.go#L665) endpoint.
+
+## `ZeroCustomFields` method
+
+`ZeroCustomFields` should return a copy of the light client with all client customizable fields with their zero value. It should not mutate the fields of the light client.
+This method is used when [scheduling upgrades](https://github.com/cosmos/ibc-go/blob/v6.0.0-rc1/modules/core/02-client/keeper/proposal.go#L89). Upgrades are used to upgrade chain specific fields. 
+In the tendermint case, this may be the chainID or the unbonding period.
+For more information about client upgrades see [the developer guide](../upgrades/developer-guide.md).
+
+## `GetTimestampAtHeight` method
+
+`GetTimestampAtHeight` must return the timestamp for the consensus state associated with the provided height.
+This value is used to facilitate timeouts by checking the packet timeout timestamp against the returned value.
+
+## `Initialize` method
+
+Clients must validate the initial consensus state, and may store any client-specific metadata necessary.
+
+`Initialize` gets called when a [client is created](https://github.com/cosmos/ibc-go/blob/main/modules/core/02-client/keeper/client.go#L32).
+
+## `VerifyMembership` method
+
+`VerifyMembership` must verify the existence of a value at a given CommitmentPath at the specified height. For more information about membership proofs
+see [the proof docs](./proofs.md).
+
+## `VerifyNonMembership` method
+
+`VerifyNonMembership` must verify the absence of a value at a given CommitmentPath at a specified height. For more information about non membership proofs
+see [the proof docs](./proofs.md).
+
+## `VerifyClientMessage` method
+
+VerifyClientMessage must verify a ClientMessage. A ClientMessage could be a Header, Misbehaviour, or batch update.
+It must handle each type of ClientMessage appropriately. Calls to CheckForMisbehaviour, UpdateState, and UpdateStateOnMisbehaviour
+will assume that the content of the ClientMessage has been verified and can be trusted. An error should be returned
+if the ClientMessage fails to verify.
+
+## `CheckForMisbehaviour` method
+
+Checks for evidence of a misbehaviour in Header or Misbehaviour type. It assumes the ClientMessage
+has already been verified.