Refactor docs

docs/portable-mode.md

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+# Portable mode
+
+SFTPGo allows to share a single directory on demand using the `portable` subcommand:
+
+```
+sftpgo portable --help
+To serve the current working directory with auto generated credentials simply use:
+
+sftpgo portable
+
+Please take a look at the usage below to customize the serving parameters
+
+Usage:
+  sftpgo portable [flags]
+
+Flags:
+  -C, --advertise-credentials           If the SFTP service is advertised via multicast DNS, this flag allows to put username/password inside the advertised TXT record
+  -S, --advertise-service               Advertise SFTP service using multicast DNS (default true)
+  -d, --directory string                Path to the directory to serve. This can be an absolute path or a path relative to the current directory (default ".")
+  -f, --fs-provider int                 0 means local filesystem, 1 Amazon S3 compatible, 2 Google Cloud Storage
+      --gcs-automatic-credentials int   0 means explicit credentials using a JSON credentials file, 1 automatic (default 1)
+      --gcs-bucket string
+      --gcs-credentials-file string     Google Cloud Storage JSON credentials file
+      --gcs-key-prefix string           Allows to restrict access to the virtual folder identified by this prefix and its contents
+      --gcs-storage-class string
+  -h, --help                            help for portable
+  -l, --log-file-path string            Leave empty to disable logging
+  -p, --password string                 Leave empty to use an auto generated value
+  -g, --permissions strings             User's permissions. "*" means any permission (default [list,download])
+  -k, --public-key strings
+      --s3-access-key string
+      --s3-access-secret string
+      --s3-bucket string
+      --s3-endpoint string
+      --s3-key-prefix string            Allows to restrict access to the virtual folder identified by this prefix and its contents
+      --s3-region string
+      --s3-storage-class string
+  -s, --sftpd-port int                  0 means a random non privileged port
+  -c, --ssh-commands strings            SSH commands to enable. "*" means any supported SSH command including scp (default [md5sum,sha1sum,cd,pwd])
+  -u, --username string                 Leave empty to use an auto generated value
+```
+
+In portable mode, SFTPGo can advertise the SFTP service and, optionally, the credentials via multicast DNS, so there is a standard way to discover the service and to automatically connect to it.
+
+Here is an example of the advertised service including credentials as seen using `avahi-browse`:
+
+```
+= enp0s31f6 IPv4 SFTPGo portable 53705                         SFTP File Transfer   local
+   hostname = [p1.local]
+   address = [192.168.1.230]
+   port = [53705]
+   txt = ["password=EWOo6pJe" "user=user" "version=0.9.3-dev-b409523-dirty-2019-10-26T13:43:32Z"]
+```
+