crawler: add ability to limit cache size

README.md

@@ -316,6 +316,13 @@ will be cleared every time the program is run). Path can be defined with
 - `CACHE=mysql`: MySQL cache. Connection can be defined with the following
 environment variables: `MYSQL_USER`, `MYSQL_PWD`, `MYSQL_DB`, `MYSQL_HOST`
 
+To limit the siz of the cache:
+' `CACHE_SIZE` sets the target number of items in the cache (further items will
+be deleted but the cache migth be temporarily bigger than that). Defaults to 10k
+entries.
+- `CACHE_LIFESPAN` sets how often the cache must be trimmed (i.e. cut down to
+the number of items set in `CACHE_SIZE`). Defaults to 1hr.
+
 ## Configuration
 ### Length limitation