Add doc about caching

Hi

I'm using mapcontrib as UI for OpenPianosMap. But something isn't clear since the beginning: how cache is working ? This project is a worldwide map, so the request to Overpass API is quite huge and caching results is necessary.

But I noticed that new added POIs that matches criterias of Overpass request are never reflected in map (even some weeks after).

So I'm guessing: the cache is "static"; it's never automatically updated, isn't it ? I sometimes manually "update the cache" by:

• disable the cache for the 3 layers of my map
• zoom out and refresh the page to trigger the Overpass request
• re-enable caching for each layer

If I'm right (there's no automatic cache update), it would be great to have an option to automatically update cached data via a customizable delay (let the user choose the number of days before cache update)

What do you think about this ? (I'll create another issue for this if you confirm that there's no automatic cache update)

Thanks !