[#1015] docs: Add subnetwork creation HOWTO

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

cmd/neofs-adm/docs/subnetwork-creation.md

@@ -0,0 +1,41 @@
+# NeoFS subnetwork creation
+
+This is a short guide on how to create NeoFS subnetworks. This guide 
+considers that side chain and inner ring (alphabet nodes) have already 
+been deployed and side chain contains deployed `subnet` contract.
+
+## Prerequisites
+
+To follow this guide you need:
+- neo-go side chain RPC endpoint;
+- latest released version of [neofs-adm](https://github.com/nspcc-dev/neofs-node/releases);
+- wallet with NeoFS account.
+
+## Creation
+
+```shell
+$ neofs-adm morph subnet create \
+    -r <side_chain_RPC_endpoint> \
+    -w </path/to/owner/wallet> \
+    --notary
+Create subnet request sent successfully. ID: 4223489767.
+```
+
+**NOTE:** use `--notary` only in notary-enabled environmental. You need to
+have sufficient notary deposit (not expired with enough GAS balance). This
+is the only one command that requires alphabet signature and, therefore,
+the only one command that needs `--notary` flag. Your subnet ID will differ
+from the example.
+
+Default account in the wallet that was passed with `-w` flag is the owner
+of the just created subnetwork.
+
+You can check if your subnetwork was created successfully:
+
+```shell
+$ neofs-adm morph subnet get \
+    -r <side_chain_RPC_endpoint> \
+    --subnet <subnet_ID>
+Owner: NUc734PMJXiqa2J9jRtvskU3kCdyyuSN8Q
+```
+Your owner will differ from the example.