Improve JSON-LD representation for Address and add it to the docs

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

docs/contribute/activity_pub.md

@@ -111,3 +111,61 @@ Example:
   "url": "http://mobilizon1.com/events/8cf76e9f-c426-4912-9cd6-c7030b969611"
 }
 ```
+
+#### location
+
+We use Schema.org's `location` property on `Event`.
+[The ActivityStream vocabulary to represent places](https://www.w3.org/TR/activitystreams-vocabulary/#places) is quite limited so instead of using `Place` from ActivityStreams we use `Place` from Schema.org.
+
+A [Schema.org `Place` type](https://schema.org/Place) has [an `address` property](https://schema.org/address), which we assume to be [of `PostalAddress` type](https://schema.org/PostalAddress) and [a `geo` property](https://schema.org/geo), which is assumed to be of [`GeoCoordinates` type](https://schema.org/GeoCoordinates).
+
+```json
+{
+  "@context": [
+    "...",
+    {
+      "GeoCoordinates": "sc:GeoCoordinates",
+      "Place": "sc:Place",
+      "PostalAddress": "sc:PostalAddress",
+      "address": {
+        "@id": "sc:address",
+        "@type": "sc:PostalAddress"
+      },
+      "addressCountry": "sc:addressCountry",
+      "addressLocality": "sc:addressLocality",
+      "addressRegion": "sc:addressRegion",
+      "geo": {
+        "@id": "sc:geo",
+        "@type": "sc:GeoCoordinates"
+      },
+      "location": {
+        "@id": "sc:location",
+        "@type": "sc:Place"
+      },
+      "postalCode": "sc:postalCode",
+      "sc": "http://schema.org#",
+      "streetAddress": "sc:streetAddress",
+    }
+  ],
+  "id": "http://mobilizon2.com/events/945f350d-a3e6-4bcd-9bf2-0bd2e4d353c5",
+  "location": {
+      "address": {
+        "addressCountry": "France",
+        "addressLocality": "Lyon",
+        "addressRegion": "Auvergne-Rhône-Alpes",
+        "postalCode": "69007",
+        "streetAddress": "10 Rue Jangot",
+        "type": "PostalAddress"
+      },
+      "geo": {
+        "latitude": 4.8425657,
+        "longitude": 45.7517141,
+        "type": "GeoCoordinates"
+      },
+      "id": "http://mobilizon2.com/address/bdf7fb53-7177-46f3-8fb3-93c25a802522",
+      "name": "10 Rue Jangot",
+      "type": "Place"
+    },
+  "type": "Event"
+}
+```