Add logger docs

Author: Илья

README.md

@@ -545,3 +545,27 @@ Methods of the query builder:
 - `txControl(\Ydb\Table\TransactionControl $tx_control)` - transaction control with custom settings
 
 You can chain these methods for convenience.
+
+## Logging
+
+For logging purposes, you need use class, which implements `\Psr\Log\LoggerInterface`.
+Example of using:
+
+```php
+class SimpleLogger extends \Psr\Log\LoggerInterface
+{
+    use LoggerTrait;
+    public function log($level, string $message, array $context = []): void
+    {
+        fwrite(STDERR,
+            date("d/m/y H:i:s")." ".self::getLevelName($level). " ".$message." ".json_encode($context)."\n"
+        );
+    }
+}
+
+$config = [
+    'logger' => new SimpleLogger()
+]
+$ydb = new \YdbPlatform\Ydb\Ydb($config);
+```
+

src/Ydb.php

@@ -109,12 +109,12 @@ public function __construct($config = [], LoggerInterface $logger = null)
 
         if (!is_null($logger) && isset($config['logger'])){
             throw new \Exception('Logger set in 2 places');
-        } else if (isset($config['logger']) && $config['logger'] instanceof LoggerInterface) {
-            $this->logger = $config['logger'];
+        } else if (isset($config['logger'])) {
+            $this->setLogger($config['logger']);
         } else if ($logger) {
-            $this->logger = $logger;
+            $this->setLogger($logger);
         } else {
-            $this->logger = new NullLogger();
+            $this->setLogger(new NullLogger());
         }
 
         $this->retry = new Retry($this->logger);
@@ -363,4 +363,9 @@ public function getLogger()
     {
         return $this->logger;
     }
+
+    protected function setLogger(LoggerInterface $logger){
+        $this->logger = $logger;
+    }
+
 }