Add docs

server/README.md

@@ -30,6 +30,9 @@ Server-side implementation for shoes-lxd-multi
 - `LXD_MULTI_OVER_COMMIT_PERCENT`
     - Percent of able over commit in CPU
     - default: `100`
+- `LXD_MULTI_RESOURCE_CACHE_PERIOD_SEC`
+    - Period of cache resource in seconds
+    - default: `10`
 
 ## Note
 LXD Server can't use `zfs` in storageclass if use `--privileged`. ref: https://discuss.linuxcontainers.org/t/docker-with-overlay-driver-in-lxd-cluster-not-working/9243