chore: update contribution guide (#4028)

* chore: update contribution guide

* refactor: content-parity with doc contribution guide

* fix: remove double double-dashes and add npm cmd

CONTRIBUTING.md

@@ -1,14 +1,72 @@
 # Contributing to Nuxt.js
 
+First of all, thank you for considering to contribute to Nuxt.js! :heart:
+
+## Reporting Issues
+
+A great way to contribute to the project is to send a detailed report when you encounter an issue.
+To make things easier for contributors and maintainers, we use [CMTY](https://cmty.app/nuxt/issues/new?repo=nuxt.js).
+
+Please make sure to include a reproduction repository or [CodeSandBox](https://codesandbox.io/s/github/nuxt/codesandbox-nuxt/tree/master/)
+so that bugs can be reproduced without great efforts. The better a bug can be reproduced, the faster we can start fixing it!
+
+## Pull Requests
+
+We'd love to see your pull requests, even if it's just to fix a typo!
+
+However, any significant improvement should be associated to an existing
+[feature request](https://cmty.app/nuxt/issues/feature-request?repo=nuxt.js)
+or [bug report](https://cmty.app/nuxt/issues/bug-report?repo=nuxt.js).
+
+### Getting started
+
 1. [Fork](https://help.github.com/articles/fork-a-repo/) this repository to your own GitHub account and then [clone](https://help.github.com/articles/cloning-a-repository/) it to your local device.
-2. Run `npm install` to install the dependencies.
+2. Run `npm install` or `yarn install` to install the dependencies.
 
 > _Note that both **npm** and **yarn** have been seen to miss installing dependencies. To remedy that, you can either delete the `node_modules` folder in your example app and install again or do a local install of the missing dependencies._
 
-3. Add an example app to `examples/` before writing any code.
+> If you are adding a dependency, please use `yarn add`. The `yarn.lock` file is the source of truth for all Nuxt dependencies.
-4. Create a fixture app under `test/fixtures/`. See others for examples.
 
-To run an example or fixture app with your copy of Nuxt, do:
+### Test structure
+
+A great PR, whether it includes a bug fix or a new feature, will often include tests.
+To write great tests, let us explain our test structure:
+
+#### Fixtures
+
+The fixtures (found under `tests/fixtures`) contain several Nuxt applications. To keep build time as short as possible,
+we don't build an own Nuxt application per test. Instead, the fixtures are built (`yarn test:fixtures`) before running
+the actual unit tests.
+
+Please make sure to **alter** or **add a new fixture** when submitting a PR to reflect the changes properly (if applicable).
+
+Also, don't forget to **rebuild** a fixture after changing it by running the corresponding test
+with `jest test/fixtures/my-fixture/my-fixture.test.js`!
+
+#### Unit tests
+
+The unit tests can be found in `tests/unit` and will be executed after building the fixtures. A fresh Nuxt server will be used
+per test so that no shared state (except the initial state from the build step) is present.
+
+After adding your unit tests, you can run them directly:
+
+```sh
+jest test/unit/test.js
+```
+
+Or you can run the whole unit test suite:
+
+```sh
+yarn test:unit
+```
+
+Again, please be aware that you might have to rebuild your fixtures before!
+
+### Testing your changes
+
+While working on your PR you will likely want to check if your fixture is set up correctly or debug your current changes.
+
+To do so you can use the Nuxt script itself to launch for example your fixture or an example app:
 
 ```sh
 bin/nuxt examples/your-app
@@ -17,36 +75,32 @@ bin/nuxt test/fixtures/your-fixture-app
 
 > `npm link` could also (and does, to some extent) work for this, but it has been known to exhibit some issues. That is why we recommend calling `bin/nuxt` directly to run examples.
 
-Once you've modified Nuxt and seen your modifications reflected correctly in your example app and fixture, add `unit` and, if necessary, `e2e` tests that can ensure its functionality and future maintanability. Study other tests carefully for reference. 
+### Examples
 
-## Running specific tests
+If you are working on a larger feature, please set up an example app in `examples/`.
+This will help greatly in understanding changes and also help Nuxt users to understand the feature you've built in-depth.
 
-```sh
+### Linting
-npm install jest -g
-jest test/unit/test.js
-```
 
-## Running test suites
+As you might have noticed already, we are using ESLint to enforce a code standard. Please run `yarn lint` before committing
+your changes to verify that the code style is correct. If not, you can use `yarn lint --fix` or `npm run lint -- --fix` (no typo!) to fix most of the
+style changes. If there are still errors left, you must correct them manually.
 
-```sh
+### Documentation
-yarn test
-```
 
-Or, per group:
+If you are adding a new feature, do a refactoring or change the behavior of Nuxt in any other manner, you'll likely
+want to document the changes. Please do so with a PR to the [docs](https://github.com/nuxt/docs/pulls) repository.
+You don't have to write documentation up immediately (but please do so as soon as your pull request is mature enough).
 
-```sh
+### Final checklist
-yarn test:fixtures
-yarn test:unit
-yarn test:e2e
-```
 
-## Debugging tests on macOS
+When submitting your PR, there is a simple template that you have to fill out.
+Please tick all appropriate "answers" in the checklists.
+
+### Troubleshooting
+
+#### Debugging tests on macOS
 
 Searching for `getPort()` will reveal it's used to start new Nuxt processes during tests. It's been seen to stop working on macOS at times and may require you to manually set a port for testing.
 
 Another common issue is Nuxt processes that may hang in memory when running fixture tests. A ghost process will often prevent subsequent tests from working. Run `ps aux | grep -i node` to inspect any hanging test processes if you suspect this is happening.
-
-## Opening PRs
-
-- Please make sure your PR passes the lint test (`npm run lint`).
-- Please wait for tests to run and verify potential failures.