docs: add section on testing local changes (#8802)

* docs: add section on testing local changes

* chore: use the root command for watching

* Update .github/CONTRIBUTING.md

Co-authored-by: Almeida <almeidx@pm.me>

* Update .github/CONTRIBUTING.md

Co-authored-by: Jiralite <33201955+Jiralite@users.noreply.github.com>

* Update .github/CONTRIBUTING.md

Co-authored-by: Jiralite <33201955+Jiralite@users.noreply.github.com>

* Update .github/CONTRIBUTING.md

Co-authored-by: Jiralite <33201955+Jiralite@users.noreply.github.com>

Co-authored-by: Almeida <almeidx@pm.me>
Co-authored-by: Jiralite <33201955+Jiralite@users.noreply.github.com>
Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

.github/CONTRIBUTING.md

@@ -17,6 +17,25 @@ To get ready to work on the codebase, please do the following:
 5. Run `yarn test` to run ESLint and ensure any JSDoc changes are valid
 6. [Submit a pull request](https://github.com/discordjs/discord.js/compare) (Make sure you follow the [conventional commit format](https://github.com/discordjs/discord.js/blob/main/.github/COMMIT_CONVENTION.md))
 
+## Testing changes locally
+
+If you want to test changes you've made locally, you can do so by using `yarn link`. This will create a symlink to your local copy of the discord.js libraries.
+
+1. Create a new directory `mkdir discordjs-test` and move into it `cd discordjs-test`
+2. Initialize a new yarn 3 project `yarn init -2`
+3. Disable pnp `yarn config set nodeLinker node-modules`
+4. Now link the local discord.js project you cloned earlier `yarn link -A {PATH_TO_DISCORDJS_REPO}`
+5. Install packages you'd like to test locally `yarn add discord.js@latest`, `yarn add @discordjs/rest@latest`, etc. **Note: Make sure you use `latest` tag or else yarn will try to install the remote package from npm**
+6. Import the package in your source code and test them out!
+
+### Working with TypeScript packages
+
+When testing local changes, you may notice you need to manually recompile TypeScript projects on every change in order to get the latest code changes to test locally.
+
+To avoid this you can use the `--watch` parameter in the package build script to automatically recompile the project when changes are detected.
+
+For example, to automatically recompile the `@discordjs/rest` project when changes are detected, run `yarn turbo run build --filter=@discordjs/rest -- --watch` in the root folder of where you cloned the discord.js repo.
+
 ## Adding new packages
 
 If you'd like to create another package under the `@discordjs` organization run the following command: