Add Using GPX files with Geocoder and Geofield

Closes #2747193

README.md

@@ -355,6 +355,22 @@ enabling support and configuration for the
 - in your *settings.php add:
   `$settings['cache']['bins']['geocoder'] = 'cache.backend.permanent_database'`
 
+## Using GPX files with Geocoder and Geofield
+
+Here's how to set up [Geofield](https://www.drupal.org/project/geofield), [Geocoder](https://www.drupal.org/project/geocoder), and [Leaflet](https://www.drupal.org/project/leaflet) (optional) to display a track uploaded in .gpx file format.
+
+1. Install Geocoder, Geocoder Field, Geocoder Geofield, Geofield, and Leaflet (optional)
+1. Create a "GPX File" Geocoder provider under _/admin/config/system/geocoder/geocoder-provider_
+1. Create a file field in a content type, used to upload a GPX file
+1. Create a Geofield field in a content type, allowing only one value
+1. Set up the Geofield field like this:
+   - Geocode → Geocode from an existing field
+   - Geocode from an existing field → select file field used to upload the GPX file
+   - Geocoder plugins → checked "GPX File"
+1. For the Geofield field, in "Manage display" choose "Leaflet Map" (if available) or just "Raw Output"
+1. Create new content with an attached GPX file
+1. The Leaflet map should show the route or point, or if raw value used, something like `POINT (2.862523 42.874582)` or `MULTILINESTRING ((-3.4584857337177 48.72786023654 [...]`
+
 # Upgrading from Geocoder 2.x to 3.x (and above))
 
 ## Site builders