add validator readme

contracts/validator/README.md

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+# Validator Introduction
+
+The `Validator` contract is responsible for validating signatures. It implements the `IValidator` interface, which defines the necessary methods for signature validation.
+
+By default, SoulWallet implements the `Validator` contract with two signature verification schemas: ECDSA and passkey.
+
+Users can generate their passkeys, which can then be used for signature verification. This feature provides an additional layer of security and flexibility, allowing users to authenticate their operations in a way that best suits their needs.
+
+## Signature Validation Methods
+
+The `Validator` contract supports two main validation flows: user operation validation and ERC1271 standard signature validation for contracts.
+
+### User operation validation flow
+
+In the standard ERC4337 user operation, the entry point calls `account.validateUserOp` in the `handleOps` operation. First, the user operation signature is decoded in the `validateUserOp` function. The first 32 bytes define the validator address. If the validator address is installed in the SoulWallet contract, it will trigger a view call to the validation contract. The `validator` will then validate the correctness of the signature.
+
+### ERC1271 Standard Signature Validation
+
+The `Validator` contract also supports the ERC1271 standard for signature validation. This standard defines a `isValidSignature` function that contracts can implement to validate signatures. Similar to the user operation signature decoding, when `isValidSignature` is invoked, the initial 32 bytes define the validator address.
+
+The SoulWallet user operation signature format is as follows:
+
+- `[0:20]`: `validator address` - This is the address of the validator contract that will verify the signature.
+- `[20:24]`: `validator signature length` - This is the length of the validator signature. It is a 4-byte value, allowing for a maximum signature length of 2^32.
+- `[24:24+n]`: `validator signature` - This is the actual signature produced by the validator. Its length is determined by the `validator signature length` field.
+- `[24+n:]`: `hook signature` - This is an optional signature for a hook function that can be executed after the main operation.
+
+![validation flow](validation_flow.png)