Create feature-flag-testing.md (#1074)

* Create feature-flag-testing.md

* Working on docs

* Add instructions for creating a new bundle

* Update feature flag testing documentation

* initial docs on how to create a feature flag

Author: lukasoppermann

contributor-docs/feature-flag-testing.md

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+# Testing primitives using a feature flag
+
+## 1. Create a PR
+
+Start by creating a PR in `primer/primitives`. Once all the tests are passing you are ready to move on.
+
+You will needs the pre-release version later in the process. So copy the version number from the `Published @primer/primitives` job. This is something like `0.0.0-20241007214729`.
+
+## 2. Create the feature flag
+
+To create the feature flag, you need to log into the [DevPortal](https://devportal.githubapp.com/feature-flags) and navigate to the [feature flags](https://devportal.githubapp.com/feature-flags) section.
+
+Click on the `New feature flag` button and fill in the required information.
+
+**Important:** Make sure to choose a descriptive name that is easy to understand. You **CANNOT** change the name.
+
+Confirm with the `Save feature` button.
+
+## 3. Add yourself to the feature flag
+
+You should first add yourself to the feature flag and test that everything is working. Afterwards you can add others.
+
+To do so, open the feature flag and navigate to the `Targeting rules` tab.
+Now click the `Switch to another stamp` dropdown and select `ditcom`.
+Under `Actors` add yourself.
+
+## 4. Create a PR in `github/github`
+
+Now it is time to create a PR in `github/github` to add your feature for all users that have the feature flag active.
+
+This requires a few changes:
+
+### Preload the feature flag
+
+You need to add your feature flag to `app/controllers/application_controller/preload_feature_flags_dependency.rb`:
+
+```
+:primitives_my_feature_flag # a comment describing what the feature flag does
+```
+
+### Install your pre-release
+
+In `github/github` dependencies are installed in workspaces. Primitives are part of the `@npm-workspaces/primer` workspace.
+Additionally, since the stable version of primitives is already installed, you need to install your version with an [npm alias](https://docs.npmjs.com/cli/v8/commands/npm-install#:~:text=npm%20install%20%3Calias%3E%40npm%3A%3Cname%3E%3A).
+
+For example:
+
+```bash
+npm i @primer/primitives-my-feature-flag@npm:@primer/[email protected] --workspace=@npm-workspaces/primer
+```
+
+**NOTE:** This package needs to be checked into git. On dotcom all packages are added in git.
+
+### Create a new bundle (optional)
+
+If you want to load some new tokens you can add a new bundle and load it for users who have the feature flags enabled.
+
+To do so, first create a new bundle by creating a new folder in `app/assets/stylesheets/bundles/` for example `primer-primitives-your-bundle`.
+In this folder add an `index.scss` file that includes all files you want to load:
+
+```scss
+@import '@primer/primitives-my-feature-flag/dist/css/functional/themes/new-theme.css';
+```
+
+You now need to load your css bundle if the feature flag is active by placing the code below in all nessesary places, for example `app/views/layouts/application.html.erb`.
+
+```ruby
+# this bundle (index.scss in the folder) will only be loaded if the users has the primitives_my_feature_flag feature flag enabled
+<%= stylesheet_bundle "primer-primitives-your-bundle" if user_feature_enabled?(:primitives_my_feature_flag) %>
+```
+
+Now you need to generate the `SERVICEOWNERS` and `CODEOWNERS` by running `bin/generate-service-files.rb` in the console or by running it as a task in vs code.
+
+It should add a line like this to the `SERVICEOWNERS`:
+
+```
+app/assets/stylesheets/bundles/primer-primitives-your-bundle/ :primer_dotcom
+```
+
+And a line like this to the `CODEOWNERS`:
+
+```
+/app/assets/stylesheets/bundles/primer-primitives-your-bundle/ @github/primer-reviewers
+```