document pruning and the prune method in readme

Author: dbu

README.md

@@ -103,6 +103,18 @@ To invalidate tags, call the method `Psr6Store::invalidateTags` or use the
 `PurgeTagsListener` from the [FOSHttpCache][3] library to handle tag 
 invalidation requests.
 
+### Pruning Expired Cache Entries
+
+By default, this cache removes expired entries from the cache after every 500
+cache write operations. You can change the frequency with the `prune_threshold`
+configuration setting.
+
+You can also manually trigger pruning by calling the `prune()` method on the
+cache. With this, you could for example implement a cronjob that loads the store
+and prunes it at a configured interval, to prevent slowing down random requests
+that were cache misses because they have to wait for the pruning to happen. If you
+have set up a cron job, you should disable pruning by setting the threshold to 0.
+
 ### WARNING
 
 It is possible to configure other cache adapters or lock stores than the