Add Contributing file for #180

Author: brettz9

CONTRIBUTING.md

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+# CONTRIBUTING to eslint-plugin-jsdoc
+
+## Testing changes locally
+
+You might try a TDD approach and add tests within the `test` directory,
+to try different configs, you may find it easier to try out changes in
+a separate local directory.
+
+You can run [`npm link`](https://docs.npmjs.com/cli/link) for this purpose,
+pointing from your project to this project. For example, while in your project
+root and with `eslint-plugin-jsdoc` as a sibling, run:
+
+```shell
+npm link ../eslint-plugin-jsdoc
+```
+
+## Building the project
+
+After running `npm install` to get the latest dependencies and devDependencies,
+you can run the following command to update the `dist` files, with `dist/index.js`
+being the `main` entrance point from `package.json`:
+
+```shell
+npm run build
+```
+
+## Coding standards
+
+The project follows ESLint rules from [`canonical`](https://www.npmjs.com/package/eslint-config-canonical)
+and testing follows its subconfig, `canonical/mocha`.
+
+## Testing
+
+Tests are expected. Each rule file should be in CamelCase (despite the rule names themselves being hyphenated) and should be added within `test/assertions` and then imported/required by
+`test/rules/index.js`.
+
+Each rule file should be an ESM default export of an object which has `valid` and `invalid` array properties containing the tests. Tests of each type should be provided.
+
+`parserOptions` will be `ecmaVersion: 6` by default, but tests can override `parserOptions`
+with their own.
+
+See ESLint's [RuleTester](https://eslint.org/docs/developer-guide/nodejs-api#ruletester)
+for more on the allowable properties (e.g., `code`, `errors` (for invalid rules),
+`options`, `settings`, etc.).
+
+## Requirements for PRs
+
+PRs should be mergeable, [rebasing](https://git-scm.com/book/en/v2/Git-Branching-Rebasing)
+first against the latest `master`.
+
+The number of commits will ideally be limited in number, unless extra commits
+can better show a progression of features.
+
+Commit messages should be worded clearly and the reason for any PR made clear
+by linking to an issue or giving a full description of what it achieves.