Add comments and usage example

Dockerfile

@@ -1,13 +1,31 @@
 FROM atmoz/sftp:alpine
 MAINTAINER Glenn Y. ROLLAND <glenux@glenux.net>
 
-# docker run \
+# Usage 
-#     -v /host/users.conf:/etc/sftp/users.conf:ro \
+# =====
-#     -v mySftpVolume:/home \
-#     -p 2222:22 -d atmoz/sftp
 
+# Generate your own SSH keys
+# --------------------------
+
+# ssh-keygen -t ed25519 -f ssh_host_ed25519_key < /dev/null
+# ssh-keygen -t rsa -b 4096 -f ssh_host_rsa_key < /dev/null
+
+# Prepare user configuration
+# --------------------------
 # /host/users.conf:
 # 
 # foo:123:1001:100
 # bar:abc:1002:100
 # baz:xyz:1003:100
+
+# Run the container
+# -----------------
+# 
+# docker run \
+#     -v /host/ssh_host_ed25519_key:/etc/ssh/ssh_host_ed25519_key \
+#     -v /host/ssh_host_rsa_key:/etc/ssh/ssh_host_rsa_key \
+#     -v /host/users.conf:/etc/sftp/users.conf:ro \
+#     -v mySftpVolume:/home \
+#     -v /host/share:/home/foo/share \
+#     -p 2222:22 -d atmoz/sftp
+