chore: add note to contributing.md (#153)

Author: jmsjtu

CONTRIBUTING.md

@@ -18,3 +18,7 @@ Any code change should be submitted as a pull request. The description should ex
 ## Review process
 
 The bigger the pull request, the longer it will take to review and merge. Try to break down large pull requests in smaller chunks that are easier to review and merge. Also make sure to reference the related issues in the pull request message if any.
+
+## Eslint versions
+
+Salesforce currently supports both ESLint v8 and v9. All new rules must be written in both versions. The `master` branch is on ESLint v9, while ESLint v8 rules are maintained in the `eslint-v8` branch.