html-ids: update README with check id info

README.rst

@@ -144,6 +144,21 @@ Checks take no arguments, and must return a ``dict`` whose keys are applied to t
     def my_check():
         return {'x': 1}
 
+
+By default stacktrace buttons and modal popups on ``/status/dashboard/`` page have a slugified name as HTML id. You can also define some custom id to identify elements in HTML separately from their display names, for example when slugifying does something completely unreadable::
+
+    @check
+    def check_redis():
+        return {
+            "127.0.0.1:1234/0": {
+                "ok": True,
+                "id": "some-custom-redis-html-id",
+                # because `slugify` makes '12700112340'
+            }
+        }
+
+
+
 In the absence of any checks, a 404 is thrown, which is then handled by the
 ``json_view`` decorator.