First bash at a readme added.

readme.txt

+=== Squat Radar Calendar ===
+
+Provides integration with https://radar.squat.net/ to display events on your Wordpress website.
+
+Stable tag: 2.0.0
+License: GPLv2 or later
+License URI: https://www.gnu.org/licenses/gpl-2.0.html
+
+== Installation ==
+
+Enable the plugin as normal for your system.
+
+Once activated:
+1. Visit the Widgets page. Here you will now see 'Squat Radar Events' widget, and the 'Squat Radar Shortcode' sidebar.
+2. Add the widget to either the sidebar you want, or if you want to use the shortcode in content, the 'Squat Radar Shortcode' sidebar.
+3. To configure the sidebar go to https://radar.squat.net/events and filter for the events you want to show. Maybe your city and group, or a category etc.
+4. Once you have the events filter you want copy the address from your address bar into the widget.
+5. Select which fields you would like to show.
+
+If you put the widget in a displayed sidebar that's it. The filtered list of upcoming events will now show up.
+If you used the Shorcode sidebar, add [squat_radar_sidebar] to the content where you want the events to display.