docs: add PSR-16 and composer require instructions for session storage

Author: lvluoyue

README.md

@@ -212,6 +212,8 @@ $server = Server::builder()
     ->build();
 
 // Override with PSR-16 cache-based storage
+// Requires psr/simple-cache and symfony/cache (or any other PSR-16 implementation)
+// composer require psr/simple-cache symfony/cache
 $redisAdapter = new RedisAdapter(
     RedisAdapter::createConnection('redis://localhost:6379'),
     'mcp_sessions'

docs/server-builder.md

@@ -159,6 +159,8 @@ $server = Server::builder()
     ->build();
 
 // Override with PSR-16 cache-based storage
+// Requires psr/simple-cache and symfony/cache (or any other PSR-16 implementation)
+// composer require psr/simple-cache symfony/cache
 $redisAdapter = new RedisAdapter(
     RedisAdapter::createConnection('redis://localhost:6379'),
     'mcp_sessions'