Adds README and php plugin code

README.md

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+# WP OpenFreeMap - OpenFreeMap WordPress Plugin
+
+A [WordPress plugin](https://wordpress.org/plugins) that renders [OpenFreeMap](https://openfreemap.org) using [MapLibre GL](https://maplibre.org).
+
+## Installation
+1. Upload the plugin to your `/wp-plugins/` directory
+2. Activate the plugin through the 'Plugins' menu in WordPress
+3. Go to "Settings > OpenFreemap" to configure the plugin
+
+
+## Basic Usage
+Once activated you can render [OpenFreeMap](https://openfreemap.org) map widgets using the shortcode
+```
+[openfreemap [options...]]
+```
+The options are as follows
+- `lat`  
+   Vertical center of the map (default: `47.1092664`)
+- `lon`  
+   Horizontal center of the map (default: `12.3453356`)
+- `zoom`  
+   Initial zoom of the map (default: `16.5`)
+- `color`  
+   Marker color in CSS notation (default: `#ff0000`)
+- `height`  
+   Height of the map widget in CSS notation (default: `450px`)
+
+## Links
+- https://openfreemap.org
+- https://maplibre.org