docs(api): mention use of @<path> and @- for --field

Signed-off-by: Babak K. Shandiz <babakks@github.com>

Author: babakks

pkg/cmd/api/api.go

@@ -280,7 +280,7 @@ func NewCmdApi(f *cmdutil.Factory, runF func(*ApiOptions) error) *cobra.Command
 
 	cmd.Flags().StringVar(&opts.Hostname, "hostname", "", "The GitHub hostname for the request (default \"github.com\")")
 	cmd.Flags().StringVarP(&opts.RequestMethod, "method", "X", "GET", "The HTTP method for the request")
-	cmd.Flags().StringArrayVarP(&opts.MagicFields, "field", "F", nil, "Add a typed parameter in `key=value` format")
+	cmd.Flags().StringArrayVarP(&opts.MagicFields, "field", "F", nil, "Add a typed parameter in `key=value` format (use \"@<path>\" or \"@-\" to read value from file or stdin)")
 	cmd.Flags().StringArrayVarP(&opts.RawFields, "raw-field", "f", nil, "Add a string parameter in `key=value` format")
 	cmd.Flags().StringArrayVarP(&opts.RequestHeaders, "header", "H", nil, "Add a HTTP request header in `key:value` format")
 	cmd.Flags().StringSliceVarP(&opts.Previews, "preview", "p", nil, "Opt into GitHub API previews (names should omit '-preview')")