Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/psr/log/README.md

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+PSR Log
+=======
+
+This repository holds all interfaces/classes/traits related to
+[PSR-3](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-3-logger-interface.md).
+
+Note that this is not a logger of its own. It is merely an interface that
+describes a logger. See the specification for more details.
+
+Usage
+-----
+
+If you need a logger, you can use the interface like this:
+
+```php
+<?php
+
+use Psr\Log\LoggerInterface;
+
+class Foo
+{
+    private $logger;
+
+    public function __construct(LoggerInterface $logger = null)
+    {
+        $this->logger = $logger;
+    }
+
+    public function doSomething()
+    {
+        if ($this->logger) {
+            $this->logger->info('Doing work');
+        }
+
+        // do something useful
+    }
+}
+```
+
+You can then pick one of the implementations of the interface to get a logger.
+
+If you want to implement the interface, you can require this package and
+implement `Psr\Log\LoggerInterface` in your code. Please read the
+[specification text](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-3-logger-interface.md)
+for details.