Add screenshot attribution, fix line breaks

blockbase/readme.txt

@@ -43,12 +43,16 @@ A Universal theme is a theme that works in Full Site Editing and in the Customiz
 Blockbase WordPress Theme, (C) 2021 Automattic, Inc.
 Blockbase 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 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.
+
+Blockbase uses the following third-party resources:
+
+The Water Fan, by Winslow Homer
+License: CC0
+Source: https://www.artic.edu/artworks/38666/the-water-fan
+Included in theme screenshot.

blockbase/style.css

@@ -25,4 +25,11 @@ 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.
+
+Blockbase uses the following third-party resources:
+
+The Water Fan, by Winslow Homer
+License: CC0
+Source: https://www.artic.edu/artworks/38666/the-water-fan
+Included in theme screenshot.
 */