Issue #2575387 by chrisfree, r_sharma08, hussainweb, sudhanshug, Cottser,...

Issue #2575387 by chrisfree, r_sharma08, hussainweb, sudhanshug, Cottser, anil280988, AjitS, rashid_786, drupal.ninja03, Prashant.c, snehi, jordanpagewhite, anil.gangwal, brahmjeet789, jhodgdon, davidhernandez, Manjit.Singh: Add README.txt to Classy Base Theme?

core/themes/classy/README.txt

+ABOUT CLASSY
+-----------
+
+Classy is a base theme that provides many classes in its markup. For cleaner
+markup (fewer classes), the default Stable base theme can be used instead.
+
+To use Classy as your base theme, set the 'base theme' in your theme's .info.yml
+file to "classy":
+  base theme: classy
+
+See https://www.drupal.org/theme-guide/8/classy for more information on using
+the Classy theme.
+
+ABOUT DRUPAL THEMING
+--------------------
+
+See https://www.drupal.org/theme-guide/8 for more information on Drupal theming.