Add explicit information on how to preallocate volumes

FAQ.md

@@ -23,6 +23,19 @@ The default volume size is 30GB, and default to 8 volumes. Very likely you would
 
 SeaweedFS also assumes there is a default disk type, which is either empty or `"hdd"`. If all your disk type is `"ssd"`, you should leave the disk type as empty since the disk type basically is just a tag to group the volumes.
 
+### How do I pre-allocate one or more volumes?
+
+To pre-allocate volumes, send a request to the Master server. For example:
+
+```shell
+# To replica
+curl "http://localhost:9333/vol/grow?dataCenter=000&count=4" # returns {"count":4}
+curl "http://localhost:9333/vol/grow?rack=example-rack&count=4" # returns {"count":4}
+curl "http://localhost:9333/vol/grow?dataNode=example-node&count=4" # returns {"count":4}
+```
+
+See the [Master Server API documentation](https://github.com/seaweedfs/seaweedfs/wiki/Master-Server-API#pre-allocate-volumes) for full details (and more ways to choose replication).
+
 ### How to access the server dashboard?
 SeaweedFS has web dashboards for its different services: