chore(doc): add contribution guidelines

CONTRIBUTING.md

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+# Contributing
+
+Contributions to `@sasjs/lint` are very welcome!
+Please fill in the pull request template and make sure that your code changes are adequately covered with tests when making a PR.
+
+## Architecture
+
+This project implements a number of rules for SAS projects and code. There are three types of rules:
+_ File rules - rules applied at the file level
+_ Line rules - rules applied to each line of a file \* Path rules - rules applied to paths and file names
+
+When implementing a new rule, place it in the appropriate folder for its type.
+Please also make sure to export it from the `index.ts` file in that folder.
+
+The file for each rule typically exports an object that conforms to the `LintRule` interface.
+This means it will have a `type`, `name`, `description` and `message` at a minimum.
+
+File, line and path lint rules also have a `test` property.
+This is a function that will run a piece of logic against the supplied item and produce an array of `Diagnostic` objects.
+These objects can be used in the consuming application to display the problems in the code.
+
+With some lint rules, we can also write logic that can automatically fix the issues found.
+These rules will also have a `fix` property, which is a function that takes the original content -
+either a line or the entire contents of a file, and returns the transformed content with the fix applied.
+
+## Testing
+
+Testing is one of the most important steps when developing a new lint rule.
+It helps us ensure that our lint rules do what they are intended to do.
+
+We use `jest` for testing, and since most of the code is based on pure functions, there is little mocking to do.
+This makes `@sasjs/lint` very easy to unit test, and so there is no excuse for not testing a new rule. :)
+
+When adding a new rule, please make sure that all positive and negative scenarios are tested in separate test cases.
+When modifying an existing rule, ensure that your changes haven't affected existing functionality by running the tests on your machine.
+
+You can run the tests using `npm test`.
+
+## Code Style
+
+This repository uses `Prettier` to ensure a uniform code style.
+If you are using VS Code for development, you can automatically fix your code to match the style as follows:
+
+- Install the `Prettier` extension for VS Code.
+- Open your `settings.json` file by choosing 'Preferences: Open Settings (JSON)' from the command palette.
+- Add the following items to the JSON.
+  ```
+    "editor.formatOnSave": true,
+    "editor.formatOnPaste": true,
+  ```
+
+If you are using another editor, or are unable to install the extension, you can run `npm run lint:fix` to fix the formatting after you've made your changes.