docs: added README for msk iam module

Author: kikyomits

sasl/aws_msk_iam/README.md

@@ -0,0 +1,125 @@
+# AWS MSK IAM
+
+This extension provides a capability to get authenticated with [AWS Managed Apache Kafka](https://aws.amazon.com/msk/) 
+through AWS IAM.
+
+You can use the `Mechanism` for SASL authentication, like below.
+
+```go
+ctx := context.Background()
+
+// using aws-sdk-go-v2
+creds, err := aws.NewConfig().Credentials.Retrieve(ctx)
+if err != nil {
+    // NOTE: address error properly
+    panic(err)
+}
+m := &Mechanism{
+    GenericSigner: &AWSSignerV2{
+        Signer:      sigv2.NewSigner(),
+        Credentials: creds,
+    },
+    Region:   "us-east-1",
+    SignTime: time.Now(),
+    Expiry:   time.Minute * 5,
+}
+config := kafka.ReaderConfig{
+    Brokers:     []string{"https://localhost"},
+    GroupID:     "some-consumer-group",
+    GroupTopics: []string{"some-topic"},
+    Dialer: &kafka.Dialer{
+        Timeout:       10 * time.Second,
+        DualStack:     true,
+        SASLMechanism: m,
+        TLS:           &tls.Config{},
+    },
+}
+```
+
+
+## Examples
+
+### aws-sdk-go-v2
+
+If you use [aws-sdk-go-v2](https://github.com/aws/aws-sdk-go-v2), you can use `AWSSignerV2` to get authenticated with MSK.
+
+```go
+package main
+
+import (
+	"context"
+	"time"
+
+	"github.com/aws/aws-sdk-go-v2/aws"
+	sigv2 "github.com/aws/aws-sdk-go-v2/aws/signer/v4"
+)
+
+func main() {
+	ctx := context.Background()
+	creds, err := aws.NewConfig().Credentials.Retrieve(ctx)
+	if err != nil {
+		// NOTE: address error properly
+		panic(err)
+	}
+	m := &Mechanism{
+		GenericSigner: &AWSSignerV2{
+			Signer:      sigv2.NewSigner(),
+			Credentials: creds,
+		},
+		Region:   "us-east-1",
+		SignTime: time.Now(),
+		Expiry:   time.Minute * 5,
+	}
+}
+```
+
+### aws-sdk-go
+
+If you use [aws-sdk-go](https://github.com/aws/aws-sdk-go), which is a old version of SDK, you can use `AWSSignerV1` to get authenticated with MSK.
+
+```go
+package main
+
+import (
+    "time"
+    
+    "github.com/aws/aws-sdk-go/aws/defaults"
+    sig "github.com/aws/aws-sdk-go/aws/signer/v4"
+)
+
+func main() {
+	m := &Mechanism{
+		GenericSigner: &AWSSignerV1{
+			Signer: sig.NewSigner(defaults.Config().Credentials),
+		},
+		Region:   "us-east-1",
+		SignTime: time.Now(),
+		Expiry:   time.Minute * 5,
+	}
+}
+```
+
+### Authentication method at Old Version of Kafka-Go
+
+The old versions of `kafka-go` library was using [aws-sdk-go](https://github.com/aws/aws-sdk-go). 
+We keep this functionality for backward compatibility.
+
+```go
+package main
+
+import (
+	"time"
+
+	"github.com/aws/aws-sdk-go/aws/defaults"
+	sig "github.com/aws/aws-sdk-go/aws/signer/v4"
+)
+
+func main() {
+	m := &Mechanism{
+		Signer:   sig.NewSigner(defaults.Config().Credentials),
+		Region:   "us-east-1",
+		SignTime: time.Now(),
+		Expiry:   time.Minute * 5,
+	}
+}
+```

sasl/aws_msk_iam/msk_iam.go

@@ -35,7 +35,7 @@ type SignerIfc interface {
 // Mechanism implements sasl.Mechanism for the AWS_MSK_IAM mechanism, based on the official java implementation:
 // https://github.com/aws/aws-msk-iam-auth
 type Mechanism struct {
-	// Deprecated, to support both of the aws-sdk-go-v1 and aws-sdk-go-v2, we implemented GenericSigner. The sig.Signer to use when signing the request.
+	//  \The sig.Signer to use when signing the request. To support both of the aws-sdk-go-v1 and aws-sdk-go-v2, we implemented GenericSigner too.
 	Signer *sig.Signer
 	// interface which supports both of the aws-sdk-go-v1 and aws-sdk-go-v2, use when signing the request.
 	GenericSigner SignerIfc