Added docs

doc/Extensions/Memcached.md

@@ -0,0 +1,16 @@
+# Memcached
+
+LibreNMS can store SQL results in memcached to achieve performance advantages of in-memory value storage and removing work load for frequent queries off the MySQL backend.
+
+To enable memcached in your install you need to have `memcached` installed and the PHP extension `php5-memcached` or `php-memcached` and add the following lines to your `config.php`:
+
+```php
+$config['memcached']['enable']  = true;
+$config['memcached']['host']    = "localhost";
+$config['memcached']['port']    = 11211;
+```
+
+By default values are kept for 4 Minutes inside the memcached, you can adjust this retention time by modifying the `$config['memcached']['ttl']` value to any desired amount of seconds.
+It's strongly discouraged to set this above `300` (5 Minutes) to avoid interferences with the polling, discovery and alerting processes.
+
+If you use the Distributed Poller, you can point this to the same memcached instance. However a local memcached will perform better in any case.