docs(README): add the migration guide (#46)

* docs(README): add the migration guide

* fix typo

README.md

@@ -4,7 +4,7 @@
 [![Go](https://github.com/nabeken/aws-go-dynamodb/actions/workflows/go.yml/badge.svg)](https://github.com/nabeken/aws-go-dynamodb/actions/workflows/go.yml)
 [![MIT License](http://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/nabeken/aws-go-dynamodb/blob/master/LICENSE)
 
-`aws-go-dynamodb` is a Amazon DynamoDB utility library built with [aws/aws-sdk-go-v2](https://github.com/aws/aws-sdk-go-v2).
+`aws-go-dynamodb` is an Amazon DynamoDB utility library built with [aws/aws-sdk-go-v2](https://github.com/aws/aws-sdk-go-v2).
 
 ## v2
 
@@ -15,10 +15,50 @@ import "github.com/nabeken/aws-go-dynamodb/v2"
 
 As of Jan 27, 2024, the master branch is work-in-progress for `aws-sdk-go-v2` support. Please be aware of it.
 
+### Migration to v2
+
+`v2` has the breaking changes. Especially, you have better to test marshal and unmarshal behavior in v2 with the existing items that is created with v1.
+
+You can find the examples in [the test code](https://github.com/nabeken/aws-go-dynamodb/blob/master/table/table_test.go).
+
+**item.{Unmarshaler,Marshaler}**:
+
+The `Unmarshaler` and `Marshaler` interface in v1 have been removed in favor of the official [`attributevalue.Unmarshaler`](https://pkg.go.dev/github.com/aws/aws-sdk-go-v2/feature/dynamodb/attributevalue#Unmarshaler) and  [`attributevalue.Marshaler`](https://pkg.go.dev/github.com/aws/aws-sdk-go-v2/feature/dynamodb/attributevalue#Marshaler).
+
+**Marshaling and Unmashaling**:
+
+v2 now uses the official [`attributevalue`](https://pkg.go.dev/github.com/aws/aws-sdk-go-v2/feature/dynamodb/attributevalue) package to marshal Go value into DynamoDB item and unmarshal DynamoDB item into Go value.
+
+You have to add `dynamodbav` struct tag to make your struct work with the `attributevalue` package.
+
+**List and Set in DynamoDB**:
+
+The official `attributevalue` package handles a Go's slice as List type. If you want to handle it as Set (e.g. StringSet), you have to add `stringset` option in `dynamodbav` struct tag.
+
+**Handling of Expression Attribute**:
+
+v2's `option` package now works well with the official [`expression`](https://pkg.go.dev/github.com/aws/aws-sdk-go-v2/feature/dynamodb/expression) package.
+
+**Handling errors**:
+
+It's not the v2's specific topic but you now have to handle errors in [the way that aws-sdk-go-v2 recommends](https://aws.github.io/aws-sdk-go-v2/docs/handling-errors/).
+
+Example to check whether if a condition is failed:
+```go
+var exception *types.ConditionalCheckFailedException
+assert.True(t, errors.As(err, &exception))
+assert.Equal(t, "ConditionalCheckFailedException", exception.ErrorCode())
+```
+
 ## v1
 
 If you want to use this library with `aws-sdk-go`, please use v1 version of the library.
 
+Usage:
+```go
+import "github.com/nabeken/aws-go-dynamodb"
+```
+
 ## Testing
 
 The tests will run on DynamoDB Local running on `tcp/18000`. Docker helps you to launch it on your local.