Add a readme.txt file

This is based on the recommendation of the theme review team.

https://make.wordpress.org/themes/2015/04/29/a-revised-readme/

The goal is for a uniformly structured read file adopted by all theme
authors. Having the template ready in _s would speed up the adoption.

readme.txt

@@ -0,0 +1,27 @@
+=== _s ===
+Contributors: automattic
+Tags: featured-images, translation-ready, custom-background, theme-options
+Requires at least: 4.0
+Tested up to: 4.2
+Stable tag: 1.0.0
+License: GPLv2 or later
+License URI: http://www.gnu.org/licenses/gpl-2.0.html
+
+Short description. No more than 150 chars.
+
+== Description ==
+Full theme description.
+
+== Frequently Asked Questions ==
+
+= A question that someone might have =
+
+An answer to that question.
+
+== Changelog ==
+
+= 1.0 =
+* Initial release
+
+== Resources ==
+ * example-script.js © 2015 Jane Doe, CC0