docs of etcd config (#1622)

contrib/config/etcd/README.md

@@ -0,0 +1,41 @@
+# Etcd Config
+
+```go
+import (
+	"log"
+
+	cfg "github.com/go-kratos/kratos/contrib/config/etcd/v2"
+	"github.com/go-kratos/kratos/v2/config"
+	clientv3 "go.etcd.io/etcd/client/v3"
+	"google.golang.org/grpc"
+)
+
+// create a etcd client
+client, err := clientv3.New(clientv3.Config{
+    Endpoints:   []string{"127.0.0.1:2379"},
+    DialTimeout: time.Second,
+    DialOptions: []grpc.DialOption{grpc.WithBlock()},
+})
+if err != nil {
+    log.Fatal(err)
+}
+
+// configure the source, "path" is required
+source, err := cfg.New(client, cfg.WithPath("/app-config"), cfg.WithPrefix(true))
+if err != nil {
+    log.Fatalln(err)
+}
+
+// create a config instance with source
+c := config.New(config.WithSource(source))
+defer c.Close()
+
+// acquire config value
+foo, err := c.Value("/app-config").String()
+if err != nil {
+    log.Println(err)
+}
+println(foo)
+
+```
+