Fix pa11y issues with the map

README.md

@@ -77,6 +77,7 @@ Each entry starts at the time given, in Hamburg time, at the Hackertours desk (w
 
 - Each tour gets a marker in its colour, with its `icon` in white, and its route as a line; walking legs are dotted. Clicking a marker or a route shows the tour title, linked to the tour page, and the directions.
 - The map shows all tours at once. Scroll-wheel zoom only starts after clicking into the map, so the page still scrolls normally.
+- The map is only created when it scrolls near the visible part of the page, so visitors who don't scroll down load no map tiles. That also means the accessibility check (pa11y) doesn't see the map: it would otherwise report the text above the map, because axe wrongly takes the clipped map tiles to overlap it. Check changes to the map's markers and controls by hand, e.g. with the axe browser extension.
 - Without JavaScript, and for screen readers, it shows a list of the tours with their directions and a link to each location on openstreetmap.org.
 - If a text is missing in the page's language, the other language is used.
 - Without any tours with `geo`, the shortcode renders nothing and the build logs a warning with the ID `warning-tour-map-empty`.