diff --git a/contrib/docker/README.md b/contrib/docker/README.md
index 0da7b5662..8b1e0afee 100644
--- a/contrib/docker/README.md
+++ b/contrib/docker/README.md
@@ -84,6 +84,7 @@ Synapse specific settings:
   the Synapse instance.
 * ``SYNAPSE_ALLOW_GUEST``, set this variable to allow guest joining this server.
 * ``SYNAPSE_EVENT_CACHE_SIZE``, the event cache size [default `10K`].
+* ``SYNAPSE_CACHE_FACTOR``, the cache factor [default `0.5`].
 * ``SYNAPSE_REPORT_STATS``, set this variable to `yes` to enable anonymous
   statistics reporting back to the Matrix project which helps us to get funding.
 * ``SYNAPSE_RECAPTCHA_PUBLIC_KEY``, set this variable to the recaptcha public