docs: mention the need for worker process restarts to avoid "memory leaks" (Issue #346)

It might not be ideal to mention such details in the overview, but that seems
like the best place for generic advice.

(Kallithea should still aim for keeping memory growth under control.)

docs/overview.rst

@@ -204,6 +204,14 b' dynamically generated pages from a relat'
 also often used inside organizations with a limited amount of users and thus no
 continuous hammering from the internet.
 
+.. note::
+   Kallithea, the libraries it uses, and Python itself do in several places use
+   simple caching in memory. Caches and memory are not always released in a way
+   that is suitable for long-running processes. They might appear to be leaking
+   memory. The worker processes should thus regularly be restarted - for
+   example after 1000 requests and/or one hour. This can usually be done by the
+   web server or the tool used for running it as a system service.
+
 
 .. _Python: http://www.python.org/
 .. _Gunicorn: http://gunicorn.org/