Add graves to memorial theme

assets/layers/memorial/memorial.json

@@ -78,6 +78,9 @@
         "de": "ein Denkmal",
         "fr": "un mémorial"
       },
+      "description": {
+        "en": "A memorial is a physical object which remembers a person or event."
+      },
       "tags": [
         "historic=memorial"
       ]
@@ -218,6 +221,12 @@
           "addExtraTags": [
             "natural=tree"
           ]
+        },
+        {"if": "historic=tomb",
+          "then": {
+            "en": "This is a gravestone; the person is buried here"
+          },
+          "addExtraTags": ["memorial="]
         }
       ],
       "render": {