README: Recommend some DB settings

Author: methane

README.md

@@ -56,15 +56,31 @@ Make sure [Git is installed](https://git-scm.com/downloads) on your machine and
 _Go MySQL Driver_ is an implementation of Go's `database/sql/driver` interface. You only need to import the driver and can use the full [`database/sql`](https://golang.org/pkg/database/sql/) API then.
 
 Use `mysql` as `driverName` and a valid [DSN](#dsn-data-source-name)  as `dataSourceName`:
+
 ```go
 import "database/sql"
 import _ "github.com/go-sql-driver/mysql"
 
 db, err := sql.Open("mysql", "user:password@/dbname")
+if err != nil {
+	panic(err)
+}
+// See "Important settings" section.
+db.SetConnMaxLifetime(time.Minute * 3)
+db.SetMaxOpenConns(10)
+db.SetMaxIdleConns(10)
 ```
 
 [Examples are available in our Wiki](https://github.com/go-sql-driver/mysql/wiki/Examples "Go-MySQL-Driver Examples").
 
+### Important settings
+
+`db.SetConnMaxLifetime()` is required to ensure connections are closed by the driver sefely before connection is closed by MySQL server, OS, or other middlewares. Since some middlewares close idle connections idle by 5 minutes, we recommend timeout shorter than 5 minutes. This setting helps load balancing and changing system variables too.
+
+`db.SetMaxOpenConns()` is highly recommended to limit the number of connection used by the application. There is no recommended limit number because it depends on application and MySQL server.
+
+`db.SetMaxIdleConns()` is recommended to be set same to (or greater than) `db.SetMaxOpenConns()`. When it is smaller than `SetMaxOpenConns()`, connections can be opened and closed very frequently than you expect. Idle connections can be closed by the `db.SetConnMaxLifetime()`. If you want to close idle connections more rapidly, you can use `db.SetConnMaxIdleTime()` since Go 1.15.
+
 
 ### DSN (Data Source Name)