Add default token file path to README and usage

Fixes #15

Author: 178inaba

README.md

@@ -18,6 +18,13 @@ $ stdin | gistup
 The github gist opens automatically in the browser.  
 If the browser can not be opened, the URL will be displayed.
 
+The token is saved in the following file and omitting the user name and password entry from next time:
+
+* macOS, Linux
+  * `~/.config/gistup/token`
+* Windows
+  * `%APPDATA%\gistup\token`
+
 ### Options
 
 * `-a`

main.go

@@ -47,7 +47,14 @@ func main() {
 	log.SetPrefix(fmt.Sprintf("%s: ", os.Args[0]))
 	flag.Usage = func() {
 		fmt.Fprintf(os.Stderr, "usage: %s [-a] [-d <description>] [-insecure] [-p] [-url <api_baseurl>] <file>...\n", os.Args[0])
-		fmt.Fprintf(os.Stderr, "       stdin | %s [-a] [-d <description>] [-n <file_name>] [-p]\n", os.Args[0])
+		fmt.Fprintf(os.Stderr, "       stdin | %s [-a] [-d <description>] [-n <file_name>] [-p]\n\n", os.Args[0])
+		fmt.Fprintln(os.Stderr, "The token is saved in the following file and omitting the user name and password entry from next time:")
+		if runtime.GOOS == "windows" {
+			fmt.Fprintln(os.Stderr, "%APPDATA%\\gistup\\token")
+		} else {
+			fmt.Fprintln(os.Stderr, "~/.config/gistup/token")
+		}
+		fmt.Fprintln(os.Stderr)
 		flag.PrintDefaults()
 	}
 	flag.Parse()