Add readme.txt and theme screenshot. (#8)

readme.txt

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+=== Twenty Twenty-Two ===
+Contributors: wordpressdotorg
+Requires at least: 5.8
+Tested up to: 5.8
+Requires PHP: 5.6
+License: GPLv2 or later
+License URI: http://www.gnu.org/licenses/gpl-2.0.html
+
+== Description ==
+
+The WordPress default theme for 2022.
+
+== Copyright ==
+
+Twenty Twenty-Two WordPress Theme, 2021 WordPress.org
+Twenty Twenty-Two is distributed under the terms of the GNU GPL.
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU General Public License for more details.
+
+This theme bundles the following third-party resources:
+
+Source Serif Font
+Copyright 2014-2021 Adobe (http://www.adobe.com/)
+License: SIL Open Font License, 1.1, https://opensource.org/licenses/OFL-1.1
+Source: https://github.com/adobe-fonts/source-serif
+
+Images
+"White Bellied Flycatcher" illustrated by Elizabeth Gould. Public Domain.
+https://www.rawpixel.com/image/321474/free-illustration-image-bird-vintage-birds