add README description and example

Author: antagoony

cucumber-picocontainer/README.md

@@ -123,3 +123,29 @@ customization. If you want to customize your dependency injection context,
 it is recommended to provide your own implementation of 
 `io.cucumber.core.backend.ObjectFactory` and make it available through
 SPI.
+
+However it is possible to configure additional PicoContainer `Provider`s and/or
+`ProviderAdapter`s. For example, some step definition classes might require a
+database connection as a constructor argument.
+
+```java
+package com.example.app;
+
+import java.sql.*;
+import io.cucumber.picocontainer.PicoConfiguration;
+import org.picocontainer.injectors.ProviderAdapter;
+
+@PicoConfiguration(providerAdapters = { ExamplePicoConfiguration.DatabaseConnectionProvider.class })
+public class ExamplePicoConfiguration {
+
+    public static class DatabaseConnectionProvider extends ProviderAdapter {
+        public Connection provide() throws ClassNotFoundException, ReflectiveOperationException, SQLException {
+            // Connecting to MySQL Using the JDBC DriverManager Interface
+            // https://dev.mysql.com/doc/connector-j/en/connector-j-usagenotes-connect-drivermanager.html
+            Class.forName("com.mysql.cj.jdbc.Driver").getDeclaredConstructor().newInstance();
+            return DriverManager.getConnection("jdbc:mysql://localhost:3306/mydb", "mydbuser", "mydbpassword");
+        }
+    }
+
+}
+```