docs: add code of conduct rule

CONTRIBUTING.md

@@ -3,6 +3,7 @@
 ## General rules
 
 1. **English only** for issues & pull requests.
+2. Adhere to the [**Code of Conduct**](https://github.com/tycrek/degoogle/blob/master/CODE_OF_CONDUCT.md)
 2. **Do not edit README.md!** This file is automatically generated. Instead, use:
    - [`src/yaml/`](https://github.com/tycrek/degoogle/tree/master/src/yaml) for structured data
    - [`src/md/`](https://github.com/tycrek/degoogle/tree/master/src/md) for regular text