chore(doc): explain how to extend default rules

README.md

@@ -59,3 +59,6 @@ Here is how this GitLab CI template chooses the Gitleaks rules to use:
 1. It first looks for a `.gitleaks.toml` file at the root of your project repository.
 2. If not found, it uses the `.toml` file specified with the `$GITLEAKS_RULES` variable.
 3. If not specified, [default Gitleaks rules](https://github.com/zricethezav/gitleaks/blob/master/config/gitleaks.toml) are used.
+
+
+Your Gitleaks rule file can extend the default rules by using `[extend]` + `useDefault = true` as explained in the [Gitleaks documentation](https://github.com/gitleaks/gitleaks/blob/master/README.md#configuration)