docs: add gitlab compatibility

Signed-off-by: Cédric OLIVIER <cedric3.olivier@orange.com>

README.md

@@ -37,3 +37,7 @@ 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/default.go) are used.
+
+## Gitlab compatibility
+
+:information_source: This template is actually tested and validated on GitLab Community Edition instance version 13.12.11