Issue #2972442 by spitzialist, Mukeysh, imalabya, markconroy, cilefen,...

Issue #2972442 by spitzialist, Mukeysh, imalabya, markconroy, cilefen, volkswagenchick, Eli-T, longwave, andrewmacpherson: Add README.txt to Umami theme

core/profiles/demo_umami/themes/umami/README.txt

+ABOUT UMAMI
+-----------
+
+Umami is the theme used for the "Umami food magazine" demonstration site.
+The Umami theme uses Classy as its base theme.
+
+ABOUT DRUPAL THEMING
+--------------------
+
+See https://www.drupal.org/docs/8/theming for more information on Drupal
+theming.