Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

modules/README.txt

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+Modules extend your site functionality beyond Drupal core.
+
+WHAT TO PLACE IN THIS DIRECTORY?
+--------------------------------
+
+Placing downloaded and custom modules in this directory separates downloaded and
+custom modules from Drupal core's modules. This allows Drupal core to be updated
+without overwriting these files.
+
+DOWNLOAD ADDITIONAL MODULES
+---------------------------
+
+Contributed modules from the Drupal community may be downloaded at
+https://www.drupal.org/project/project_module.
+
+ORGANIZING MODULES IN THIS DIRECTORY
+------------------------------------
+
+You may create subdirectories in this directory, to organize your added modules,
+without breaking the site. Some common subdirectories include "contrib" for
+contributed modules, and "custom" for custom modules. Note that if you move a
+module to a subdirectory after it has been enabled, you may need to clear the
+Drupal cache so it can be found.
+
+MULTISITE CONFIGURATION
+-----------------------
+
+In multisite configurations, modules found in this directory are available to
+all sites. You may also put modules in the sites/all/modules directory, and the
+versions in sites/all/modules will take precedence over versions of the same
+module that are here. Alternatively, the sites/your_site_name/modules directory
+pattern may be used to restrict modules to a specific site instance.
+
+MORE INFORMATION
+----------------
+
+Refer to the “Developing for Drupal” section of the README.txt in the Drupal
+root directory for further information on extending Drupal with custom modules.