mysql: add README.md (#7824)

vlib/mysql/README.md

@@ -0,0 +1,30 @@
+For Linux, you need to install `MySQL development` package and `pkg-config`.
+For Windows, install [the installer](https://dev.mysql.com/downloads/installer/) ,
+then copy the `include` and `lib` folders to `<V install directory>\thirdparty\mysql`.
+
+## Basic Usage
+
+```v
+import mysql
+
+// Create connection
+mut connection := mysql.Connection{
+	username: 'root'
+	dbname: 'mysql'
+}
+// Connect to server
+connection.connect() ?
+// Change the default database
+connection.select_db('db_users') ?
+// Do a query
+get_users_query_result := connection.query('SELECT * FROM users') ?
+// Get the result as maps
+for user in get_users_query_result.maps() {
+	// Access the name of user
+	println(user['name'])
+}
+// Free the query result
+get_users_query_result.free()
+// Close the connection if needed
+connection.close()
+```