doc: mention as_regex in README

Author: Thomzoy

README.rst

@@ -194,6 +194,20 @@ Also supports pyproject.toml like:
     [tool.liccheck.authorized_packages]
     uuid = 1.30
 
+By default, exact matching is required between each package's license and one of the license of the authorized or unauthorized list.
+You can also provide regular expressions to match licenses by using the ``as_regex`` boolean flag. For instance, to exclude GPL licenses,
+one could define the following configuration in ``pyproject.toml``:
+
+::
+
+    ...
+
+    unauthorized_licenses = [
+        '\bgpl'
+    ]
+    as_regex = true
+    
+
 Using liccheck with pre-commit
 ==============================