explains #596 (#602)

* explain #596

* Update README.md

Co-authored-by: darkweak <[email protected]>

---------

Co-authored-by: darkweak <[email protected]>

Author: teodorescuserban

README.md

@@ -283,7 +283,7 @@ This system is able to invalidate by tags your cloud provider cache. Actually it
 |:--------|:------------------|:-----------------------------------------------------------|:------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | `GET`   | `/`               | -                                                          | List stored keys cache                                                                                                                                                              |
 | `GET`   | `/surrogate_keys` | -                                                          | List stored keys cache                                                                                                                                                              |
-| `PURGE` | `/{id or regexp}` | -                                                          | Purge selected item(s) depending. The parameter can be either a specific key or a regexp                                                                                            |
+| `PURGE` | `/{id or regexp}` | -                                                          | Purge selected item(s) depending. The parameter can be either a specific key or a regexp; use `$` to end a specific key; without `$`, `id` is considered a regex |
 | `PURGE` | `/?ykey={key}`    | -                                                          | Purge selected item(s) corresponding to the target ykey such as Varnish (deprecated)                                                                                                |
 | `PURGE` | `/`               | `Surrogate-Key: Surrogate-Key-First, Surrogate-Key-Second` | Purge selected item(s) belong to the target key in the header `Surrogate-Key` (see [Surrogate-Key system](https://github.com/darkweak/souin/blob/master/cache/surrogate/README.md)) |
 | `PURGE` | `/flush`          | -                                                          | Purge all providers and surrogate storages                                                                                                                                          |