docs: specify usage of the placeholder tag (#448)

Author: jeralm

Diff for: README.md

@@ -561,7 +561,7 @@ Both can coexist with standard Tag parsing.
 | `name:"X"`           | Long name, for overriding field name.                                                                                                                                                                                                                                                                                          |
 | `help:"X"`           | Help text.                                                                                                                                                                                                                                                                                                                     |
 | `type:"X"`           | Specify [named types](#custom-named-decoders) to use.                                                                                                                                                                                                                                                                          |
-| `placeholder:"X"`    | Placeholder text.                                                                                                                                                                                                                                                                                                              |
+| `placeholder:"X"`    | Placeholder input, if flag. e.g. `` `placeholder:"<the-placeholder>"` `` will show `--flag-name=<the-placeholder>` when displaying help.                                                                                                                                                                                       |
 | `default:"X"`        | Default value.                                                                                                                                                                                                                                                                                                                 |
 | `default:"1"`        | On a command, make it the default.                                                                                                                                                                                                                                                                                             |
 | `default:"withargs"` | On a command, make it the default and allow args/flags from that command                                                                                                                                                                                                                                                       |