Update README

Author: pcj

README.md

@@ -1,11 +1,27 @@
 # layer
 
-Print info about container images.
+Print info about container image layers.
+
+```
+NAME:
+   layer - inspect layers of an image
+
+USAGE:
+   layer [global options] command [command options] [arguments...]
+
+COMMANDS:
+   info     info prints info about the layers of an image
+   ls       ls prints the files of a layer
+   help, h  Shows a list of commands or help for one command
+
+GLOBAL OPTIONS:
+   --help, -h  show help (default: false)
+```
 
 ## Installation
 
 ```sh
-go install github.com/stackb/layer@latest
+$ go install github.com/stackb/layer@latest
 ```
 
 ## Usage
@@ -32,8 +48,36 @@ N  Layer                                                                    Size
 List files in an image:
 
 ```sh
-layer ls image.tar    # all layers
-layer ls -S image.tar # sorted
-layer ls image.tar 1  # first layer
-layer ls image.tar sha256:8fdc131ec4308d2b9196a38855550dc347e83cc0f47d739754ddeb6e03ac2cbe # by diff ID
+$ layer ls image.tar    # all layers
+$ layer ls -S image.tar # sorted
+$ layer ls image.tar 1  # first layer
+$ layer ls image.tar sha256:8fdc131ec4308d2b9196a38855550dc347e83cc0f47d739754ddeb6e03ac2cbe # by diff ID
+```
+
+List files in an image (ref):
+
+```sh
+$ layer ls -S index.docker.io/nginx:latest
+...
+-rw-r--r--  0 B     var/lib/ucf/hashfile
+-rw-r--r--  0 B     var/lib/ucf/registry
+-rwxr-xr-x  0 B     var/log/nginx/.wh..wh..opq
+-rwxrwxrwx  0 B     var/log/nginx/access.log
+-rwxrwxrwx  0 B     var/log/nginx/error.log
+
+--- sha256:f5ab86d69014270bcf4d5ce819b9f5c882b35527924ffdd11fecf0fc0dde81a4 ---
+Mode        Size    Name
+-rwxrwxr-x  1.2 kB  docker-entrypoint.sh
+
+--- sha256:c876aa251c80272eb01eec011d50650e1b8af494149696b80a606bbeccf03d68 ---
+Mode        Size    Name
+-rwxrwxr-x  2.0 kB  docker-entrypoint.d/10-listen-on-ipv6-by-default.sh
+
+--- sha256:7046505147d7f3edbf7c50c02e697d5450a2eebe5119b62b7362b10662899d85 ---
+Mode        Size    Name
+-rwxrwxr-x  1.0 kB  docker-entrypoint.d/20-envsubst-on-templates.sh
+
+--- sha256:b6812e8d56d65d296e21a639b786e7e793e8b969bd2b109fd172646ce5ebe951 ---
+Mode        Size    Name
+-rwxrwxr-x  4.6 kB  docker-entrypoint.d/30-tune-worker-processes.sh
 ```