Add contribution guidelines

CONTRIBUTING.md

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+# Contributing to Sanitize
+
+Contributions are welcome!
+
+Sanitize is a mature project, and our primary focus is ensuring stability and security while keeping the maintenance burden low. Before you spend time on a contribution, please read the following guidelines.
+
+- Bug fixes are always appreciated!
+
+- New features and major changes are unlikely to be accepted. Please keep this in mind before spending significant time on something.
+
+- Please include unit tests for all changes.
+
+At this time, we're not interested in contributions that expand the included default, basic, relaxed, or restricted configs.
+
+While we may improve the included configs from time to time, they're primarily intended to serve as examples or baselines on which users can base their own custom configs. If you have a need that isn't met by these configs, the best thing to do is to create a custom config for your own use.