Add documentation about redis cache configuration

Author: alexander-schranz

components/cache/adapters/redis_adapter.rst

@@ -201,6 +201,17 @@ In order to use tag-based invalidation, you can wrap your adapter in :class:`Sym
     $client = RedisAdapter::createConnection('redis://localhost');
     $cache = new RedisTagAwareAdapter($client);
 
+Configuring Redis
+~~~~~~~~~~~~~~~~~
+
+When redis is used as cache its recommended to configure the `maxmemory` and `maxmemory-policy`
+read more about this dopic in the offical `Redis LRU Cache Documentation`_. Normally the redis
+config can be found int `/etc/redis/redis.conf`. The `maxmemory` depend on the usage of the cache
+to low it could kickout things which could be still important, to high could end in using too
+much memory:
+
+    maxmemory 100mb
+    maxmemory-policy allkeys-lru
 
 .. _`Data Source Name (DSN)`: https://en.wikipedia.org/wiki/Data_source_name
 .. _`Redis server`: https://redis.io/
@@ -211,3 +222,4 @@ In order to use tag-based invalidation, you can wrap your adapter in :class:`Sym
 .. _`Predis Connection Parameters`: https://github.com/nrk/predis/wiki/Connection-Parameters#list-of-connection-parameters
 .. _`TCP-keepalive`: https://redis.io/topics/clients#tcp-keepalive
 .. _`Redis Sentinel`: https://redis.io/topics/sentinel
+.. _`Redis LRU Cache Documentation`: https://redis.io/topics/lru-cache