add note to be 777 chmod on file based caching

Author: samsonasik

user_guide_src/source/libraries/caching.rst

@@ -212,7 +212,7 @@ File-based Caching
 Unlike caching from the Output Class, the driver file-based caching
 allows for pieces of view files to be cached. Use this with care, and
 make sure to benchmark your application, as a point can come where disk
-I/O will negate positive gains by caching.
+I/O will negate positive gains by caching. This require a writable cache directory to be really writable (0777).
 
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 Memcached Caching