docs(command_tool): optimize the usage demo more clearer.

cmd/jwt/README.md

@@ -1,20 +1,32 @@
-`jwt` command-line tool
-=======================
+# `jwt` command-line tool
 
 This is a simple tool to sign, verify and show JSON Web Tokens from
 the command line.
 
 The following will create and sign a token, then verify it and output the original claims:
 
-     echo {\"foo\":\"bar\"} | ./jwt -key ../../test/sample_key -alg RS256 -sign - | ./jwt -key ../../test/sample_key.pub -alg RS256 -verify -
+- To sign a claim object
 
-Key files should be in PEM format. Other formats are not supported by this tool.
+```bash
+echo {\"foo\":\"bar\"} | ./jwt -key ../../test/sample_key -alg RS256 -sign -
+```
 
-To simply display a token, use:
+- To verify a token
 
-    echo $JWT | ./jwt -show -
+```bash
+JWT=$(echo {\"foo\":\"bar\"} | ./jwt -key ../../test/sample_key -alg RS256 -sign -)
+echo ${JWT} | ./jwt -key ../../test/sample_key.pub -alg RS256 -verify -
+```
 
-You can install this tool with the following command:
+- To simply display a token
 
-     go install github.com/dgrijalva/jwt-go/cmd/jwt
+```bash
+JWT=$(echo {\"foo\":\"bar\"} | ./jwt -key ../../test/sample_key -alg RS256 -sign -)
+echo $JWT | ./jwt -show -
+```
 
+## Installation
+
+Simply: `go install github.com/dgrijalva/cmd/jwt`
+
+Or you can download the code, compile it, and put the result wherever you like.