Add readme (#5)

Author: dmolineus

README.md

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+# Contao Restrict Get Parameters
+
+This Contao extension allows you to restrict GET parameters in the frontend by configuring a whitelist of allowed
+parameter patterns. Any GET parameters not matching the whitelist will be marked as unused and won't be processed by
+Contao.
+
+## System Requirements
+
+- `PHP 8.1` or higher
+- Contao `4.13+` or `5.3+`
+- Symfony `5.4+, 6.4+ or 7.0+`
+
+## Configuration
+
+In the Contao system settings, you can:
+
+1. Enable GET parameter restriction
+2. Configure a whitelist of allowed parameter patterns using wildcards (e.g., page*, *id, *alias*)
+
+It's also possible to use the bundle configuration:
+
+```yaml
+contao:
+    localconfig:
+      restrict_get_parameters: true
+      restrict_get_parameters_whitelist: ['param-a', 'param-b']
+```
+## Known limitations
+
+### Isotope product list
+
+This extension uses the unused get parameter feature of Contao and marks all get parameters as unused before rendering a
+page. Used get parameters on the page are marked as used later on, so in the end Contao throws an exception if any
+parameter was not used. This does not work when a page contains an isotope product list / filter, as Isotope marks all
+get parameters as used.