Issue #2612618 by anil280988, r_sharma08, Cottser, davidhernandez,...

Issue #2612618 by anil280988, r_sharma08, Cottser, davidhernandez, kaushalkishorejaiswal: Add README.txt file to Stable explain its role as a backwards compatibility layer

(cherry picked from commit d1d0c6e6)

core/themes/stable/README.txt

+
+ABOUT STABLE
+------------
+
+Stable is a base theme with minimal markup and very few classes. It functions
+as a backwards compatibility layer for Drupal 8's core markup and CSS. Stable
+allows markup and styling to evolve throughout the life of Drupal 8, while still
+providing themes a stable base for the clean, minimal markup provided by core.
+
+If you browse Stable's contents, you will find copies of all the Twig templates
+and CSS files provided by core. This ensures that changes made to core markup
+and styling do not affect themes using Stable as a base theme.
+
+Stable will be used as the base theme if no base theme is set in a theme's
+.info.yml file. To opt out of Stable you can set the base theme to false in
+your theme's .info.yml file (see the warning below before doing this):
+base theme: false
+
+Warning: Themes that opt out of using Stable will need continuous maintenance as
+core changes, so only opt out if you are prepared to keep track of those changes
+and how they affect your theme.
+
+ABOUT DRUPAL THEMING
+--------------------
+
+For more information, see Drupal.org's theming guide.
+https://www.drupal.org/theme-guide/8