Add pruning support (#12)

INSTALLING.md

@@ -42,4 +42,12 @@ Data is stored under the `/src/data` directory, and the default database name is
 
 ### Date/Time Format
 
-The default format for dates and times look like "May 28, 2023 at 3:15pm". Changing the string there will alter the display on the main page and when reading an item. Any [supported PHP date or time token](https://www.php.net/manual/en/datetime.format.php) is supported.
+The default format for dates and times look like "May 28, 2023 at 3:15pm". Changing the string there will alter the display on the main page and when reading an item. Any [supported PHP date or time token](https://www.php.net/manual/en/datetime.format.php) is supported.
+
+### Item Purging
+
+Feed Reader Central tries to keep the database tidy by purging items that have been read and are no longer required. There are three variants:
+- `Feed::PURGE_READ` purges non-bookmarked read items for a feed whenever it is refreshed. This is the most aggressive purging strategy, but it is also the only one that will not purge unread items.
+- `Feed::PURGE_BY_DAYS` purges non-bookmarked items that are older than `PURGE_NUMBER` days old. This is the default value, and `PURGE_NUMBER`'s default value is 30; items will be kept for 30 days, read or unread.
+- `Feed::PURGE_BY_COUNT` purges items to preserve at most `PURGE_NUMBER` non-bookmarked items for each feed.
+