Add caveat for Yarn 2+ and private repos

Author: dsame

docs/advanced-usage.md

@@ -247,5 +247,25 @@ steps:
 # `npm rebuild` will run all those post-install scripts for us.
 - run: npm rebuild && npm run prepare --if-present
 ```
+### Yarn2 configuration
+Yarn2 ignores both .npmrc and .yarnrc files created by the action, so before installing dependencies from the private repo it is necessary either to create or to modify existing yarnrc.yml file with `yarn config set` commands.
 
+Below there's a sample "Setup .yarnrc.yml" step to configure private github registy for `my-org` organisation.
+
+```yaml
+steps:
+- uses: actions/checkout@v3
+- uses: actions/setup-node@v3
+  with:
+    node-version: '14.x'
+- name: Setup .yarnrc.yml
+  run: |
+    yarn config set npmScopes.my-org.npmRegistryServer "https://npm.pkg.github.com"
+    yarn config set npmScopes.my-org.npmAlwaysAuth true
+    yarn config set npmScopes.my-org.npmAuthToken $NPM_AUTH_TOKEN
+  env:
+    NPM_AUTH_TOKEN: ${{ secrets.YARN_TOKEN }}
+- name: Install dependencies
+  run: yarn install --immutable
+```
 NOTE: As per https://github.com/actions/setup-node/issues/49 you cannot use `secrets.GITHUB_TOKEN` to access private GitHub Packages within the same organisation but in a different repository.