Add new readme file for the lib directory itself that specifies PSR-0.

core/lib/README.txt

@@ -0,0 +1,7 @@
+The core/lib directory is for classes provided by Drupal Core that are original
+to Drupal.  All Drupal-originated code must follow the PSR-0 naming convention
+for classes and namespaces as documented here:
+
+https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md
+
+The vendor namespace for Drupal-originated code is "Drupal".